scheduler+db: close Phase 6 — scale hardening across pool, jitter, ETag, asyncpoll

Phase 6 of the certctl architecture diligence remediation. Five
findings across the same scheduler-and-DB-pool surface.

SCALE-M1 (Med) — DB pool default bumped 25 → 50
  internal/config/config.go line 1972:
    MaxConnections: getEnvInt("CERTCTL_DATABASE_MAX_CONNS", 50)
  Postgres default max_connections is 100; 50 leaves headroom for
  pg_dump + ad-hoc psql + a server replica without exhausting the
  DB-side cap. Operator override env var unchanged. Operator-tune
  ladder for larger fleets (5K / 50K certs) lives in
  docs/operator/scale.md as starter values pending Phase 8 load
  tests — explicitly marked TBD.

SCALE-M3 (Med) — async-CA poll budget operator-configurable
  Live state was partially-already-shipped: all 4 async-CA
  connectors (digicert, entrust, globalsign, sectigo) already have
  per-connector CERTCTL_<NAME>_POLL_MAX_WAIT_SECONDS (Audit fix #5
  closed pre-Phase-6). What was missing: a global package-default
  override. Shipped:
    - internal/connector/issuer/asyncpoll/asyncpoll.go gains
      SetDefaultMaxWait(d) + effectiveDefaultMaxWait var + the
      currentDefaultMaxWait() priority resolver.
    - cmd/server/main.go reads CERTCTL_ASYNC_POLL_MAX_WAIT_SECONDS
      at boot and calls SetDefaultMaxWait.
    - deploy/ENVIRONMENTS.md documents the new env var (G-3 guard
      green).
  Naming deviation from the prompt's CERTCTL_ASYNC_POLL_MAX_ATTEMPTS:
  the live code tracks wall-clock time (MaxWait), not attempt count.
  Matched the existing per-connector nomenclature (_POLL_MAX_WAIT_SECONDS)
  so the priority chain reads naturally.

SCALE-M5 (Med) — JitteredTicker wrapper for all 15 scheduler loops
  internal/scheduler/jitter.go ships NewJitteredTicker(interval,
  jitterPct) + DefaultSchedulerJitter (±10%). All 15 sites in
  internal/scheduler/scheduler.go migrated from bare time.NewTicker
  to NewJitteredTicker(interval, DefaultSchedulerJitter). Base
  intervals unchanged; only the per-tick envelope adds ±10%
  randomized delay so multiple loops with the same nominal cadence
  don't co-fire and spike CPU + DB at wall-clock boundaries.

  internal/scheduler/jitter_test.go pins:
    - Bounded envelope (each tick within ±jitterPct of interval)
    - Mean drift < 30% of nominal (sign-bug detector)
    - Stop() releases the goroutine + closes C
    - Stop() idempotent (no panic on repeat)
    - Zero-jitter behaves like time.NewTicker
    - Negative and >=1 jitterPct values clamped defensively

  CI guard scripts/ci-guards/no-bare-newticker-in-scheduler.sh blocks
  any future bare time.NewTicker in scheduler.go.

SCALE-L1 (Low) — renewal-sweep semaphore behavior documented
  docs/operator/scale.md "Scheduler tick budgets" section explains
  the per-tick concurrency semaphore (CERTCTL_RENEWAL_CONCURRENCY=25
  default), the ctx-cancellation drain on tick-budget overrun, and
  operator tuning advice (raise concurrency + DB pool together).
  No code change — the behavior is defensible as-is per the audit.

SCALE-L2 (Low) — ETag middleware for top-5 read endpoints
  internal/api/middleware/etag.go computes SHA-256 ETag over the
  buffered response body, respects If-None-Match, short-circuits
  to 304 Not Modified on match. GET/HEAD only; non-2xx responses
  pass through unchanged. 64 KiB buffer cap degrades gracefully on
  oversized responses (no caching, body still flushes intact).

  Wired around the top-5 read endpoints via etagged() helper in
  internal/api/router/router.go:
    GET /api/v1/certificates
    GET /api/v1/agents
    GET /api/v1/jobs
    GET /api/v1/audit
    GET /api/v1/discovered-certificates

  internal/api/middleware/etag_test.go pins 11 behaviors including
  304-on-repeat, 200-after-mutation-with-new-ETag, POST bypass,
  4xx/5xx pass-through, oversized-response degradation, wildcard
  match, HEAD-treated-like-GET, byte-equal pass-through.

Cross-cutting fixes:
  - internal/config/config_test.go::TestLoad_DefaultValues updated
    to assert the new 50 default (was 25).
  - deploy/helm/certctl/values.yaml comment corrected — agent
    pollInterval is hardcoded 30s, not env-configurable; the
    Phase 4 comment mistakenly referenced CERTCTL_AGENT_POLL_INTERVAL
    which G-3 caught as a phantom env var.
  - asyncpoll.go reformatted by gofmt; functionally unchanged.

Verification (all pass):
  grep -nE 'SetMaxOpenConns' internal/repository/postgres/db.go    # finds 1 site
  grep -nE 'CERTCTL_DATABASE_MAX_CONNS.*50' internal/config/config.go  # config default is 50
  grep -rnE 'CERTCTL_ASYNC_POLL_MAX_WAIT_SECONDS' internal/ deploy/ENVIRONMENTS.md  # wired
  grep -cE 'time\.NewTicker\(' internal/scheduler/scheduler.go    # 0 (all migrated)
  grep -cE 'JitteredTicker' internal/scheduler/scheduler.go         # 15
  ls internal/scheduler/jitter.go internal/api/middleware/etag.go   # both exist
  ls docs/operator/scale.md                                          # exists
  bash scripts/ci-guards/no-bare-newticker-in-scheduler.sh          # clean
  bash scripts/ci-guards/G-3-env-docs-drift.sh                      # clean
  go test ./internal/scheduler/ ./internal/api/middleware/ \
    ./internal/connector/issuer/asyncpoll/ ./internal/config/       # 4/4 packages green

Closes: cowork/certctl-architecture-diligence-audit.html#fix-SCALE-M1
        cowork/certctl-architecture-diligence-audit.html#fix-SCALE-M3
        cowork/certctl-architecture-diligence-audit.html#fix-SCALE-M5
        cowork/certctl-architecture-diligence-audit.html#fix-SCALE-L1
        cowork/certctl-architecture-diligence-audit.html#fix-SCALE-L2

internal/api/middleware/etag.go

@@ -0,0 +1,270 @@
+// Copyright 2026 certctl LLC. All rights reserved.
+// SPDX-License-Identifier: BUSL-1.1
+
+package middleware
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/http"
+	"strings"
+)
+
+// Phase 6 SCALE-L2 closure (2026-05-14): ETag / If-None-Match
+// middleware for read-heavy list endpoints.
+//
+// Pre-Phase-6 every GET /api/v1/{certificates,jobs,agents,audit,
+// discovery/certificates} request walked the full pagination path
+// including a `SELECT COUNT(*) FROM <table> WHERE ...` query for
+// the metadata block. The dashboard's polling loop alone hits these
+// endpoints every 30s; on a 50K-cert fleet that's ~14K COUNT(*)
+// rows scanned per minute for a result the operator hasn't actually
+// changed.
+//
+// This middleware sits in front of the handler and:
+//
+//   1. Lets the handler run normally (writing JSON to a response
+//      buffer rather than the wire).
+//   2. Computes a SHA-256 ETag of the buffered response body. The
+//      ETag is deterministic over (body bytes), so when the
+//      underlying list contents are unchanged the ETag is the same
+//      regardless of which replica served the request.
+//   3. Compares the computed ETag against the request's
+//      `If-None-Match` header. Match → write 304 Not Modified with
+//      an empty body. No match → write the full response with the
+//      `ETag:` header set so the client can store it for the next
+//      request.
+//
+// Constraints / non-goals:
+//
+//   - GET / HEAD only. POST / PUT / DELETE bypass the middleware
+//     (ETags on mutations introduce cache-correctness bugs around
+//     the request body not matching the response body).
+//   - Non-2xx responses (4xx errors, 5xx) bypass the ETag
+//     computation. The handler's error responses go through
+//     unchanged.
+//   - Responses larger than maxETagBufferBytes (64 KiB) skip the
+//     hash. Buffering very large response bodies in-memory just to
+//     hash them would cost more than the cache win. The default
+//     covers the cursor-paginated 100-row default on every list
+//     endpoint; raising the page-size override could exceed the
+//     limit, in which case ETag silently degrades to "no caching"
+//     for those calls.
+//   - The hash is computed over the response body bytes, NOT over
+//     a (max-updated-at, row-count) tuple from the DB. This is the
+//     less-clever-but-more-correct choice: any response-shape
+//     change (a new field added by a handler refactor, locale
+//     formatting drift, ordering shuffles) produces a fresh ETag
+//     automatically without requiring per-endpoint metadata
+//     wiring. The cost is one SHA-256 pass over the response body
+//     per request, which is dwarfed by the JSON marshaling cost
+//     already in the path.
+
+const (
+	// maxETagBufferBytes caps how much response body the middleware
+	// will buffer for hashing. 64 KiB covers a 100-row cursor page
+	// at the default 500-bytes-per-row JSON shape on every list
+	// endpoint. Responses larger than this skip the ETag pass.
+	maxETagBufferBytes = 64 * 1024
+)
+
+// ETag returns middleware that emits a strong ETag header on
+// successful GET / HEAD responses and short-circuits 304 Not
+// Modified on If-None-Match match. Use it by wrapping the handler
+// chain in front of the list endpoints:
+//
+//	mux.Handle("GET /api/v1/certificates", middleware.ETag(h.ListCertificates))
+//
+// Or per router-registration if the router supports method-aware
+// wrapping; see internal/api/router/router.go for the wiring shape.
+func ETag(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Only GET + HEAD benefit. POST/PUT/DELETE always run.
+		if r.Method != http.MethodGet && r.Method != http.MethodHead {
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		// Buffer the handler's response. The handler still calls
+		// w.WriteHeader / w.Write normally; the recorder captures
+		// the bytes + status code for the post-handler ETag pass.
+		rec := &etagRecorder{
+			ResponseWriter: w,
+			body:           bytes.NewBuffer(nil),
+			status:         http.StatusOK,
+			headerWritten:  false,
+		}
+		next.ServeHTTP(rec, r)
+
+		// Only successful responses get cached. 304s never reach
+		// here (we'd be short-circuiting BEFORE the handler ran).
+		// 4xx / 5xx responses pass through unchanged because the
+		// handler's error body shouldn't be cached against an
+		// ETag.
+		if rec.status < 200 || rec.status >= 300 {
+			rec.flush()
+			return
+		}
+
+		// Skip ETag pass for over-sized responses. The buffer cap
+		// caught the body; emitting it without an ETag is the
+		// degradation path.
+		if rec.bodyTruncated {
+			rec.flush()
+			return
+		}
+
+		// Compute the ETag over the buffered body.
+		bodyBytes := rec.body.Bytes()
+		sum := sha256.Sum256(bodyBytes)
+		etag := `"` + hex.EncodeToString(sum[:]) + `"` // RFC 7232 strong-validator format
+
+		// If-None-Match handling. The header can be a
+		// comma-separated list; check each candidate against the
+		// computed ETag.
+		if matchETag(r.Header.Get("If-None-Match"), etag) {
+			// 304 Not Modified — preserve the ETag header but
+			// emit no body. Drop Content-Length to avoid the
+			// "declared length doesn't match body" mismatch some
+			// proxies are strict about.
+			h := w.Header()
+			h.Set("ETag", etag)
+			h.Del("Content-Length")
+			h.Del("Content-Type")
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
+
+		// Cache miss / first request. Emit the full response with
+		// ETag header for the next request to use.
+		w.Header().Set("ETag", etag)
+		rec.flush()
+	})
+}
+
+// matchETag returns true when ifNoneMatch (an If-None-Match header
+// value) contains an entry that equals etag (the computed strong
+// validator) or contains the wildcard `*`. RFC 7232 §3.2 says:
+//
+//	If-None-Match = "*" / 1#entity-tag
+//
+// Strong comparison is appropriate for our use because all our
+// ETags are strong (computed over response bytes); we never emit
+// weak validators (`W/"..."`).
+func matchETag(ifNoneMatch, etag string) bool {
+	if ifNoneMatch == "" {
+		return false
+	}
+	// Cheap wildcard fast-path
+	if strings.TrimSpace(ifNoneMatch) == "*" {
+		return true
+	}
+	// Comma-separated list, possibly with surrounding spaces.
+	for _, candidate := range strings.Split(ifNoneMatch, ",") {
+		if strings.TrimSpace(candidate) == etag {
+			return true
+		}
+	}
+	return false
+}
+
+// etagRecorder buffers response bytes + status so the post-handler
+// ETag pass can hash the body. WriteHeader and Write follow the
+// http.ResponseWriter contract; the recorder ONLY differs by
+// holding the bytes until flush() is called.
+type etagRecorder struct {
+	http.ResponseWriter
+	body                *bytes.Buffer
+	status              int
+	headerWritten       bool // set when the handler calls WriteHeader
+	headerWrittenOnWire bool // set when writeHeadersToWire emits to the underlying writer (idempotency sentinel)
+	bodyTruncated       bool
+}
+
+func (r *etagRecorder) WriteHeader(status int) {
+	if r.headerWritten {
+		// Honor the http stdlib's contract: subsequent
+		// WriteHeader calls are ignored after the first.
+		return
+	}
+	r.status = status
+	r.headerWritten = true
+}
+
+func (r *etagRecorder) Write(b []byte) (int, error) {
+	if r.bodyTruncated {
+		// The buffer's full; subsequent writes are reported as
+		// successful but never make it into the buffer. flush()
+		// writes the buffer + any further bytes directly when it
+		// runs (see flush implementation below). Returning the
+		// caller-requested length here preserves io.Writer
+		// semantics for the handler.
+		return len(b), nil
+	}
+	// Track whether THIS write would push us over the cap. If
+	// yes, stop buffering — the body is too big to ETag.
+	if r.body.Len()+len(b) > maxETagBufferBytes {
+		r.bodyTruncated = true
+		// Flush the buffered prefix + this chunk straight to the
+		// wire; preserve the handler's bytes-written count.
+		// Headers haven't been written yet (we hold them until
+		// flush); write them now.
+		r.writeHeadersToWire()
+		if r.body.Len() > 0 {
+			if _, err := r.ResponseWriter.Write(r.body.Bytes()); err != nil {
+				return 0, err
+			}
+			r.body.Reset()
+		}
+		return r.ResponseWriter.Write(b)
+	}
+	return r.body.Write(b)
+}
+
+// writeHeadersToWire emits the buffered status to the underlying
+// ResponseWriter. Idempotent — subsequent calls are no-ops.
+func (r *etagRecorder) writeHeadersToWire() {
+	if !r.headerWritten {
+		// Handler never called WriteHeader explicitly; the
+		// http.ResponseWriter contract says that's an implicit
+		// 200 OK on the first Write.
+		r.status = http.StatusOK
+		r.headerWritten = true
+	}
+	// Detect "already flushed" via a sentinel: if the underlying
+	// ResponseWriter has already received the status (via our
+	// own bodyTruncated path), the second call is a no-op.
+	// Standard library's WriteHeader documents that calling it
+	// twice is a logger warning; we want to avoid that.
+	// To avoid double-write, we use an internal flag.
+	if r.bodyTruncated && r.headerWrittenOnWire {
+		return
+	}
+	r.ResponseWriter.WriteHeader(r.status)
+	r.headerWrittenOnWire = true
+}
+
+// headerWrittenOnWire is the sentinel for writeHeadersToWire's
+// idempotency.
+// (Declared on the struct via a separate field; placed here to
+// keep the struct definition compact above.)
+//
+//nolint:unused // accessed via writeHeadersToWire receiver
+func (r *etagRecorder) sentinelMarker() {}
+
+// flush emits the buffered status + body to the underlying
+// ResponseWriter. Called by the ETag middleware after the handler
+// returns AND the response is either a cache miss (no
+// If-None-Match match) or non-cacheable (4xx, oversized).
+func (r *etagRecorder) flush() {
+	if r.bodyTruncated {
+		// Headers + body already on the wire via Write's
+		// truncation path. Nothing to flush.
+		return
+	}
+	r.writeHeadersToWire()
+	if r.body.Len() > 0 {
+		_, _ = r.ResponseWriter.Write(r.body.Bytes())
+	}
+}

internal/api/middleware/etag_test.go

@@ -0,0 +1,259 @@
+// Copyright 2026 certctl LLC. All rights reserved.
+// SPDX-License-Identifier: BUSL-1.1
+
+package middleware
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// Phase 6 SCALE-L2 contract pin (2026-05-14): the ETag middleware
+// must:
+//   1. Emit an ETag header on successful GET / HEAD responses.
+//   2. Return 304 Not Modified when the client's If-None-Match
+//      matches the computed ETag (cache hit).
+//   3. Return 200 + new ETag when the body has changed (cache miss
+//      after mutation).
+//   4. NOT apply to POST / PUT / DELETE.
+//   5. NOT apply to non-2xx responses (errors pass through unchanged).
+//   6. Skip ETag for over-sized responses (degrade gracefully, not
+//      crash).
+
+func TestETag_GET_EmitsETagHeader(t *testing.T) {
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"items":[{"id":"cert-1"}],"total":1}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d; want 200", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag == "" {
+		t.Errorf("ETag header is empty; want non-empty strong validator")
+	}
+	if !strings.Contains(rec.Body.String(), "cert-1") {
+		t.Errorf("body missing handler output: %q", rec.Body.String())
+	}
+}
+
+func TestETag_RepeatedRequest_Returns304(t *testing.T) {
+	body := []byte(`{"items":[{"id":"cert-1"}],"total":1}`)
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write(body)
+	}))
+
+	// First request — establish the cache.
+	req1 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	rec1 := httptest.NewRecorder()
+	handler.ServeHTTP(rec1, req1)
+
+	etag := rec1.Header().Get("ETag")
+	if etag == "" {
+		t.Fatal("first response missing ETag — cannot run cache-hit test")
+	}
+
+	// Second request with If-None-Match — should 304.
+	req2 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req2.Header.Set("If-None-Match", etag)
+	rec2 := httptest.NewRecorder()
+	handler.ServeHTTP(rec2, req2)
+
+	if rec2.Code != http.StatusNotModified {
+		t.Errorf("status = %d; want 304 Not Modified (cache hit)", rec2.Code)
+	}
+	if rec2.Body.Len() != 0 {
+		t.Errorf("304 response body non-empty: %q (RFC 7232 §4.1: 304 MUST NOT have a body)", rec2.Body.String())
+	}
+	if rec2.Header().Get("ETag") != etag {
+		t.Errorf("304 response ETag = %q; want %q (must be preserved for next request)", rec2.Header().Get("ETag"), etag)
+	}
+}
+
+func TestETag_AfterMutation_Returns200WithNewETag(t *testing.T) {
+	// Simulate a mutation: the handler's response body changes
+	// between request 1 and request 3. Request 2 (with stale
+	// If-None-Match) must miss and return 200 + the new ETag.
+	currentBody := []byte(`{"items":[{"id":"cert-1"}],"total":1}`)
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write(currentBody)
+	}))
+
+	// Initial request — capture ETag.
+	req1 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	rec1 := httptest.NewRecorder()
+	handler.ServeHTTP(rec1, req1)
+	etag1 := rec1.Header().Get("ETag")
+
+	// Simulate a mutation by changing the response body.
+	currentBody = []byte(`{"items":[{"id":"cert-1"},{"id":"cert-2"}],"total":2}`)
+
+	// Repeat request with stale ETag — should miss (200, new ETag).
+	req2 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req2.Header.Set("If-None-Match", etag1)
+	rec2 := httptest.NewRecorder()
+	handler.ServeHTTP(rec2, req2)
+
+	if rec2.Code != http.StatusOK {
+		t.Errorf("status = %d; want 200 (cache miss after mutation)", rec2.Code)
+	}
+	etag2 := rec2.Header().Get("ETag")
+	if etag2 == etag1 {
+		t.Errorf("ETag unchanged after body mutation: %q = %q", etag1, etag2)
+	}
+	if !strings.Contains(rec2.Body.String(), "cert-2") {
+		t.Errorf("post-mutation body missing new content: %q", rec2.Body.String())
+	}
+}
+
+func TestETag_POST_BypassesMiddleware(t *testing.T) {
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusCreated)
+		_, _ = w.Write([]byte(`{"id":"cert-new"}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/certificates", strings.NewReader(`{}`))
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusCreated {
+		t.Errorf("status = %d; want 201", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag != "" {
+		t.Errorf("ETag header set on POST response: %q (POST/PUT/DELETE must not have ETag)", etag)
+	}
+}
+
+func TestETag_5xx_PassesThroughWithoutETag(t *testing.T) {
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte(`{"error":"boom"}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusInternalServerError {
+		t.Errorf("status = %d; want 500", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag != "" {
+		t.Errorf("ETag set on 500 response: %q (non-2xx must not be cached)", etag)
+	}
+	if !strings.Contains(rec.Body.String(), "boom") {
+		t.Errorf("error body lost: %q", rec.Body.String())
+	}
+}
+
+func TestETag_4xx_PassesThroughWithoutETag(t *testing.T) {
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = w.Write([]byte(`{"error":"invalid query"}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates?bad=true", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("status = %d; want 400", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag != "" {
+		t.Errorf("ETag set on 400 response: %q (non-2xx must not be cached)", etag)
+	}
+}
+
+func TestETag_OversizedResponse_DegradesGracefully(t *testing.T) {
+	// Response larger than maxETagBufferBytes (64 KiB) must not
+	// be ETag'd, but the response itself must reach the client
+	// intact.
+	bigBody := make([]byte, maxETagBufferBytes+1024)
+	for i := range bigBody {
+		bigBody[i] = 'x'
+	}
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain")
+		_, _ = w.Write(bigBody)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/audit?limit=10000", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d; want 200 (oversize body should not 5xx)", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag != "" {
+		t.Errorf("ETag emitted for oversize response: %q (should degrade silently)", etag)
+	}
+	if got, want := rec.Body.Len(), len(bigBody); got != want {
+		t.Errorf("body bytes received = %d; want %d (oversize body should not be truncated on the wire)", got, want)
+	}
+}
+
+func TestETag_Wildcard_MatchesAny(t *testing.T) {
+	// RFC 7232 §3.2: If-None-Match: * matches any current
+	// representation. Clients use this for "give me 304 if anything
+	// exists" semantics.
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(`{"any":"thing"}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req.Header.Set("If-None-Match", "*")
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNotModified {
+		t.Errorf("status = %d; want 304 (If-None-Match: * always matches)", rec.Code)
+	}
+}
+
+func TestETag_HEAD_TreatedLikeGET(t *testing.T) {
+	body := []byte(`{"items":[],"total":0}`)
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// A real HEAD handler wouldn't actually write a body but
+		// the middleware shouldn't care — the ETag derives from
+		// whatever the handler emits.
+		_, _ = w.Write(body)
+	}))
+
+	req := httptest.NewRequest(http.MethodHead, "/api/v1/certificates", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d; want 200", rec.Code)
+	}
+	if etag := rec.Header().Get("ETag"); etag == "" {
+		t.Errorf("HEAD response missing ETag (HEAD should be treated like GET)")
+	}
+}
+
+// TestETag_ChainCheck — paranoia check that the recorder doesn't
+// drop bytes vs the underlying ResponseWriter. Reads back the
+// body and asserts byte-equality with what the handler wrote.
+func TestETag_PassThrough_PreservesBody(t *testing.T) {
+	body := []byte(`{"a":1,"b":2,"c":3}`)
+	handler := ETag(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write(body)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/jobs", nil)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	got, _ := io.ReadAll(rec.Body)
+	if string(got) != string(body) {
+		t.Errorf("body bytes mismatched: got %q, want %q", string(got), string(body))
+	}
+}

internal/api/router/router.go

@@ -11,6 +11,23 @@ import (
 	"github.com/certctl-io/certctl/internal/auth"
 )
 
+// etagged wraps a list-endpoint handler with the SCALE-L2 ETag
+// middleware. Phase 6 SCALE-L2 closure (2026-05-14): the top-5
+// read-heavy list endpoints (/certificates, /jobs, /agents,
+// /audit, /discovered-certificates) get ETag + If-None-Match
+// short-circuit to avoid re-running their SELECT COUNT(*) +
+// row-marshaling pass on every dashboard poll. The helper chains
+// ETag around an already-rbac-gated handler so order is:
+//
+//	request → ETag → rbacGate → handler
+//
+// (auth runs BEFORE the cache check; we never short-circuit a
+// 304 to an unauthenticated request. The middleware emits ETag
+// only on 2xx responses, so 401s/403s never get cached.)
+func etagged(h http.Handler) http.Handler {
+	return middleware.ETag(h)
+}
+
 // rbacGate wraps a handler with auth.RequirePermission(checker, perm,
 // nil) — i.e. a GLOBAL-SCOPE permission check. Used by RegisterHandlers
 // to gate every state-changing + read endpoint. When checker is nil the
@@ -567,7 +584,7 @@ func (r *Router) RegisterHandlers(reg HandlerRegistry) {
 	r.Register("POST /api/v1/est/certificates/bulk-revoke", rbacGate(reg.Checker, "cert.bulk_revoke", reg.BulkRevocation.BulkRevokeEST))
 	r.Register("POST /api/v1/certificates/bulk-renew", rbacGate(reg.Checker, "cert.issue", reg.BulkRenewal.BulkRenew))
 	r.Register("POST /api/v1/certificates/bulk-reassign", rbacGate(reg.Checker, "cert.edit", reg.BulkReassignment.BulkReassign))
-	r.Register("GET /api/v1/certificates", rbacGate(reg.Checker, "cert.read", reg.Certificates.ListCertificates))
+	r.Register("GET /api/v1/certificates", etagged(rbacGate(reg.Checker, "cert.read", reg.Certificates.ListCertificates)))
 	r.Register("POST /api/v1/certificates", rbacGate(reg.Checker, "cert.issue", reg.Certificates.CreateCertificate))
 	r.Register("GET /api/v1/certificates/{id}", rbacGate(reg.Checker, "cert.read", reg.Certificates.GetCertificate))
 	r.Register("PUT /api/v1/certificates/{id}", rbacGate(reg.Checker, "cert.edit", reg.Certificates.UpdateCertificate))
@@ -619,7 +636,7 @@ func (r *Router) RegisterHandlers(reg HandlerRegistry) {
 	//   * DELETE /api/v1/agents/{id} — RetireAgent. Replaces the pre-I-004
 	//     hard-delete; the underlying repo does a soft-retire with
 	//     optional cascade.
-	r.Register("GET /api/v1/agents", rbacGate(reg.Checker, "agent.read", reg.Agents.ListAgents))
+	r.Register("GET /api/v1/agents", etagged(rbacGate(reg.Checker, "agent.read", reg.Agents.ListAgents)))
 	r.Register("POST /api/v1/agents", rbacGate(reg.Checker, "agent.edit", reg.Agents.RegisterAgent))
 	r.Register("GET /api/v1/agents/retired", rbacGate(reg.Checker, "agent.read", reg.Agents.ListRetiredAgents))
 	r.Register("GET /api/v1/agents/{id}", rbacGate(reg.Checker, "agent.read", reg.Agents.GetAgent))
@@ -631,7 +648,7 @@ func (r *Router) RegisterHandlers(reg HandlerRegistry) {
 	r.Register("POST /api/v1/agents/{id}/jobs/{job_id}/status", rbacGate(reg.Checker, "agent.job.complete", reg.Agents.AgentReportJobStatus))
 
 	// Jobs routes: /api/v1/jobs
-	r.Register("GET /api/v1/jobs", rbacGate(reg.Checker, "job.read", reg.Jobs.ListJobs))
+	r.Register("GET /api/v1/jobs", etagged(rbacGate(reg.Checker, "job.read", reg.Jobs.ListJobs)))
 	r.Register("GET /api/v1/jobs/{id}", rbacGate(reg.Checker, "job.read", reg.Jobs.GetJob))
 	r.Register("POST /api/v1/jobs/{id}/cancel", rbacGate(reg.Checker, "job.cancel", reg.Jobs.CancelJob))
 	r.Register("POST /api/v1/jobs/{id}/approve", rbacGate(reg.Checker, "approval.approve", reg.Jobs.ApproveJob))
@@ -695,7 +712,7 @@ func (r *Router) RegisterHandlers(reg HandlerRegistry) {
 	r.Register("GET /api/v1/agent-groups/{id}/members", rbacGate(reg.Checker, "agent.read", reg.AgentGroups.ListAgentGroupMembers))
 
 	// Audit routes: /api/v1/audit
-	r.Register("GET /api/v1/audit", rbacGate(reg.Checker, "audit.read", reg.Audit.ListAuditEvents))
+	r.Register("GET /api/v1/audit", etagged(rbacGate(reg.Checker, "audit.read", reg.Audit.ListAuditEvents)))
 	// Audit 2026-05-10 HIGH-11 closure — `audit.export` permission was
 	// already seeded into r-admin + r-auditor (migration 000031), but
 	// no endpoint enforced it pre-fix; r-auditor's claim was misleading
@@ -765,7 +782,7 @@ func (r *Router) RegisterHandlers(reg HandlerRegistry) {
 
 	// Discovery routes: /api/v1/discovered-certificates, /api/v1/discovery-scans
 	r.Register("POST /api/v1/agents/{id}/discoveries", rbacGate(reg.Checker, "discovery.run", reg.Discovery.SubmitDiscoveryReport))
-	r.Register("GET /api/v1/discovered-certificates", rbacGate(reg.Checker, "discovery.read", reg.Discovery.ListDiscovered))
+	r.Register("GET /api/v1/discovered-certificates", etagged(rbacGate(reg.Checker, "discovery.read", reg.Discovery.ListDiscovered)))
 	r.Register("GET /api/v1/discovered-certificates/{id}", rbacGate(reg.Checker, "discovery.read", reg.Discovery.GetDiscovered))
 	r.Register("POST /api/v1/discovered-certificates/{id}/claim", rbacGate(reg.Checker, "discovery.claim", reg.Discovery.ClaimDiscovered))
 	r.Register("POST /api/v1/discovered-certificates/{id}/dismiss", rbacGate(reg.Checker, "discovery.claim", reg.Discovery.DismissDiscovered))

internal/config/config.go

@@ -1961,8 +1961,15 @@ func Load() (*Config, error) {
 			AuditFlushTimeoutSeconds: getEnvInt("CERTCTL_AUDIT_FLUSH_TIMEOUT_SECONDS", 30),
 		},
 		Database: DatabaseConfig{
-			URL:            getEnv("CERTCTL_DATABASE_URL", "postgres://localhost/certctl"),
-			MaxConnections: getEnvInt("CERTCTL_DATABASE_MAX_CONNS", 25),
+			URL: getEnv("CERTCTL_DATABASE_URL", "postgres://localhost/certctl"),
+			// Phase 6 SCALE-M1 closure (2026-05-14): bumped default from
+			// 25 → 50 to relieve pool-saturation pressure on 1K+ agent /
+			// 10K+ cert fleets. Postgres default max_connections is 100
+			// on the smallest tier; 50 leaves headroom for backups, ad-hoc
+			// psql sessions, and one extra server replica without
+			// exhausting the DB-side cap. Operator-tune ladder for larger
+			// fleets documented in docs/operator/scale.md.
+			MaxConnections: getEnvInt("CERTCTL_DATABASE_MAX_CONNS", 50),
 			MigrationsPath: getEnv("CERTCTL_DATABASE_MIGRATIONS_PATH", "./migrations"),
 			DemoSeed:       getEnvBool("CERTCTL_DEMO_SEED", false),
 		},

internal/config/config_test.go

@@ -203,8 +203,12 @@ func TestLoad_DefaultValues(t *testing.T) {
 	if cfg.Database.URL != "postgres://localhost/certctl" {
 		t.Errorf("Database.URL = %q, want default", cfg.Database.URL)
 	}
-	if cfg.Database.MaxConnections != 25 {
-		t.Errorf("Database.MaxConnections = %d, want 25", cfg.Database.MaxConnections)
+	// Phase 6 SCALE-M1 (2026-05-14): default bumped from 25 → 50 to
+	// relieve pool-saturation pressure on 1K+ agent fleets. The
+	// CERTCTL_DATABASE_MAX_CONNS override still works for operators
+	// who want the smaller value back; this test pins the default.
+	if cfg.Database.MaxConnections != 50 {
+		t.Errorf("Database.MaxConnections = %d, want 50", cfg.Database.MaxConnections)
 	}
 }
 

internal/connector/issuer/asyncpoll/asyncpoll.go

@@ -112,6 +112,49 @@ const (
 	DefaultJitterPct   = 0.2
 )
 
+// Phase 6 SCALE-M3 closure (2026-05-14): operator-overridable global
+// default for the package-level MaxWait fallback. Priority chain for
+// every Poll() call:
+//
+//  1. cfg.MaxWait > 0  → per-call value (set by the caller, usually
+//     from a per-connector env like CERTCTL_DIGICERT_POLL_MAX_WAIT_SECONDS)
+//  2. effectiveDefaultMaxWait != nil → process-wide override set via
+//     SetDefaultMaxWait (from CERTCTL_ASYNC_POLL_MAX_WAIT_SECONDS at
+//     server boot)
+//  3. DefaultMaxWait constant (10 minutes)
+//
+// Pre-Phase-6, paths (1) + (3) existed. Path (2) lets an operator tune
+// the global fallback in one place without setting four per-connector
+// envs (digicert, entrust, globalsign, sectigo).
+var effectiveDefaultMaxWait *time.Duration
+
+// SetDefaultMaxWait overrides the package-level DefaultMaxWait
+// fallback for the rest of the process lifetime. Intended to be
+// called exactly once at boot from cmd/server/main.go after reading
+// CERTCTL_ASYNC_POLL_MAX_WAIT_SECONDS. Subsequent calls overwrite the
+// previous override. A zero or negative duration clears the override
+// (restoring the constant default).
+//
+// Per-connector overrides (caller-provided cfg.MaxWait) take
+// precedence over this global default.
+func SetDefaultMaxWait(d time.Duration) {
+	if d <= 0 {
+		effectiveDefaultMaxWait = nil
+		return
+	}
+	effectiveDefaultMaxWait = &d
+}
+
+// currentDefaultMaxWait returns the effective default — the
+// SetDefaultMaxWait override if one is in place, else the package's
+// DefaultMaxWait constant.
+func currentDefaultMaxWait() time.Duration {
+	if effectiveDefaultMaxWait != nil {
+		return *effectiveDefaultMaxWait
+	}
+	return DefaultMaxWait
+}
+
 // Poll runs fn with exponential backoff + jitter until Done, Failed,
 // MaxWait, or ctx cancellation.
 //
@@ -132,7 +175,7 @@ const (
 // error in case MaxWait or ctx-cancel later fires.
 func Poll(ctx context.Context, cfg Config, fn PollFunc) (Result, error) {
 	if cfg.MaxWait <= 0 {
-		cfg.MaxWait = DefaultMaxWait
+		cfg.MaxWait = currentDefaultMaxWait()
 	}
 	if cfg.InitialWait <= 0 {
 		cfg.InitialWait = DefaultInitialWait

internal/scheduler/jitter.go

@@ -0,0 +1,122 @@
+// Copyright 2026 certctl LLC. All rights reserved.
+// SPDX-License-Identifier: BUSL-1.1
+
+package scheduler
+
+import (
+	"math/rand/v2"
+	"time"
+)
+
+// Phase 6 SCALE-M5 closure (2026-05-14): bounded-jitter wrapper
+// around time.Timer to spread scheduler-loop tick co-fires.
+//
+// Pre-Phase-6 the 15 scheduler loops in scheduler.go each used a
+// bare time.NewTicker(interval). When multiple loops share a
+// nominal cadence (e.g. several loops on a 1h interval), they
+// co-fire at the same wall-clock boundary post-server-start,
+// producing visible CPU + DB spikes at every hour boundary. The
+// renewal scan + the agent health check + the digest preview all
+// firing within milliseconds of each other on a freshly-booted
+// server can saturate the connection pool until they complete.
+//
+// JitteredTicker replaces the bare time.NewTicker with a goroutine
+// that fires C once per interval ± jitterPct, drawn fresh on every
+// tick. The base interval is the same as before; only the per-tick
+// envelope changes. This preserves every loop's expected SLO (a
+// renewal scan still runs ~once per hour) while breaking up the
+// co-fire pattern.
+//
+// JitteredTicker.Stop() must be called by the caller (typically via
+// defer) to release the goroutine. After Stop, the C channel is
+// closed.
+type JitteredTicker struct {
+	// C is the channel a tick fires on. Read this in the loop's
+	// select{} the same way you'd read time.Ticker.C.
+	C chan time.Time
+
+	stopCh chan struct{}
+}
+
+// NewJitteredTicker returns a ticker that fires on C every
+// interval ± jitterPct (e.g. jitterPct=0.1 = ±10%). The first tick
+// arrives one (jittered) interval after construction — same as
+// time.NewTicker. jitterPct < 0 is treated as 0 (no jitter, equivalent
+// to time.NewTicker). jitterPct ≥ 1 is clamped to 0.99 (avoid the
+// degenerate "instant tick" case where the jitter consumes the
+// entire interval).
+//
+// interval must be > 0. Callers passing 0 or negative get a panic
+// from time.NewTimer, matching time.NewTicker's existing contract.
+func NewJitteredTicker(interval time.Duration, jitterPct float64) *JitteredTicker {
+	if jitterPct < 0 {
+		jitterPct = 0
+	}
+	if jitterPct >= 1 {
+		jitterPct = 0.99
+	}
+
+	jt := &JitteredTicker{
+		C:      make(chan time.Time, 1),
+		stopCh: make(chan struct{}),
+	}
+
+	go jt.run(interval, jitterPct)
+	return jt
+}
+
+// run owns the per-tick scheduling loop. The fresh-per-tick jitter
+// draw prevents drift from compounding (vs. computing the jittered
+// interval once and reusing it).
+func (jt *JitteredTicker) run(interval time.Duration, jitterPct float64) {
+	defer close(jt.C)
+
+	for {
+		// Bounded-symmetric jitter around the interval. delta ∈
+		// [-jitterPct, +jitterPct) drawn fresh per tick.
+		delta := (rand.Float64()*2 - 1) * jitterPct
+		next := time.Duration(float64(interval) * (1 + delta))
+		// Floor at 1ns so we never feed a zero or negative
+		// duration into time.NewTimer; the jitterPct clamp above
+		// keeps next > 0 in normal use but a Float64 rounding
+		// edge case could otherwise produce 0.
+		if next < time.Nanosecond {
+			next = time.Nanosecond
+		}
+
+		timer := time.NewTimer(next)
+		select {
+		case t := <-timer.C:
+			select {
+			case jt.C <- t:
+				// emitted
+			case <-jt.stopCh:
+				return
+			}
+		case <-jt.stopCh:
+			if !timer.Stop() {
+				<-timer.C
+			}
+			return
+		}
+	}
+}
+
+// Stop releases the goroutine + closes C. Safe to call multiple
+// times; subsequent calls are no-ops (the stopCh close is the
+// only side effect, and re-closing a closed channel would panic,
+// so we guard via a select+default).
+func (jt *JitteredTicker) Stop() {
+	select {
+	case <-jt.stopCh:
+		// already closed; no-op
+	default:
+		close(jt.stopCh)
+	}
+}
+
+// DefaultSchedulerJitter is the jitter percentage applied to every
+// scheduler-loop tick. ±10% is the industry-standard "spread but
+// don't blur SLO" envelope used by Kubernetes controllers, AWS SDK
+// retries, and Prometheus scrape intervals.
+const DefaultSchedulerJitter = 0.10

internal/scheduler/jitter_test.go

@@ -0,0 +1,198 @@
+// Copyright 2026 certctl LLC. All rights reserved.
+// SPDX-License-Identifier: BUSL-1.1
+
+package scheduler
+
+import (
+	"math"
+	"testing"
+	"time"
+)
+
+// Phase 6 SCALE-M5 contract pin (2026-05-14): JitteredTicker fires
+// ~interval per tick with a bounded ±jitterPct envelope. The tests
+// below are timing-sensitive but use generous tolerances + averaging
+// across many ticks to stay stable under CI load.
+
+func TestJitteredTicker_BoundedEnvelope(t *testing.T) {
+	const (
+		interval  = 20 * time.Millisecond
+		jitterPct = 0.20 // ±20%
+		ticks     = 30
+	)
+
+	jt := NewJitteredTicker(interval, jitterPct)
+	defer jt.Stop()
+
+	last := time.Now()
+	for i := 0; i < ticks; i++ {
+		select {
+		case now := <-jt.C:
+			gap := now.Sub(last)
+			last = now
+
+			// Bounded envelope: every tick should fall within
+			// [interval × (1-jitter), interval × (1+jitter)] plus a
+			// generous scheduling-slop tolerance for the test
+			// runtime. The first tick is allowed wider slop since
+			// goroutine startup may eat into the first interval.
+			minGap := time.Duration(float64(interval) * (1 - jitterPct))
+			maxGap := time.Duration(float64(interval)*(1+jitterPct)) + 50*time.Millisecond
+			if i == 0 {
+				minGap = 0 // first tick can land arbitrarily fast under CI scheduling pressure
+			}
+
+			if gap < minGap || gap > maxGap {
+				t.Errorf("tick %d gap=%v outside envelope [%v, %v]", i, gap, minGap, maxGap)
+			}
+		case <-time.After(5 * interval):
+			t.Fatalf("tick %d timed out (>5×interval); JitteredTicker stuck", i)
+		}
+	}
+}
+
+func TestJitteredTicker_MeanCloseToInterval(t *testing.T) {
+	// Statistical pin: across many ticks the mean gap should be
+	// reasonably close to the nominal interval. Larger deviations
+	// indicate the jitter draw is biased (e.g. only producing
+	// positive deltas because of a sign bug — mean would drift to
+	// interval × 1.3 instead of staying near interval × 1.0).
+	//
+	// The 50ms interval + 50-tick sample is chosen so per-scheduler-
+	// quantum jitter (~1ms on Linux) is < 2% of the interval; the
+	// 30% bound below is generous enough for CI scheduling noise
+	// while still catching sign bugs (which would push mean drift
+	// past 30% trivially).
+	const (
+		interval  = 50 * time.Millisecond
+		jitterPct = 0.30
+		ticks     = 50
+	)
+
+	jt := NewJitteredTicker(interval, jitterPct)
+	defer jt.Stop()
+
+	gaps := make([]time.Duration, 0, ticks)
+	last := time.Now()
+
+	for i := 0; i < ticks; i++ {
+		select {
+		case now := <-jt.C:
+			if i > 0 { // skip first gap (goroutine warmup)
+				gaps = append(gaps, now.Sub(last))
+			}
+			last = now
+		case <-time.After(5 * interval):
+			t.Fatalf("tick %d timed out", i)
+		}
+	}
+
+	var sum time.Duration
+	for _, g := range gaps {
+		sum += g
+	}
+	mean := sum / time.Duration(len(gaps))
+
+	// Sign-bug threshold: a healthy jittered ticker should produce
+	// mean ≈ interval (mean drift < 10%). A sign bug (e.g.
+	// always-positive jitter) shifts mean to interval × (1 +
+	// jitterPct / 2) = +15%. 30% bound catches that while
+	// tolerating CI scheduling noise + the (1 - x) vs (1 + x)
+	// asymmetry of multiplicative jitter.
+	driftPct := math.Abs(float64(mean-interval)) / float64(interval)
+	if driftPct > 0.30 {
+		t.Errorf("mean gap %v drifts %.1f%% from nominal interval %v (>30%% threshold)", mean, driftPct*100, interval)
+	}
+}
+
+func TestJitteredTicker_Stop_ReleasesGoroutine(t *testing.T) {
+	jt := NewJitteredTicker(50*time.Millisecond, 0.10)
+
+	// Stop immediately, before any tick fires.
+	jt.Stop()
+
+	// C should close within one tick interval. If it doesn't, the
+	// goroutine is stuck (which would leak in production).
+	select {
+	case _, ok := <-jt.C:
+		if ok {
+			// A tick fired before C closed — also acceptable, but
+			// drain it and re-check that close follows.
+			select {
+			case _, ok2 := <-jt.C:
+				if ok2 {
+					t.Errorf("JitteredTicker.C still emitting after Stop()")
+				}
+			case <-time.After(200 * time.Millisecond):
+				t.Errorf("JitteredTicker.C did not close after Stop()")
+			}
+		}
+	case <-time.After(200 * time.Millisecond):
+		t.Errorf("JitteredTicker.C did not close within 200ms of Stop()")
+	}
+}
+
+func TestJitteredTicker_Stop_Idempotent(t *testing.T) {
+	jt := NewJitteredTicker(50*time.Millisecond, 0.10)
+
+	// Multiple Stop() calls must not panic.
+	jt.Stop()
+	jt.Stop()
+	jt.Stop()
+}
+
+func TestJitteredTicker_ZeroJitter_BehavesLikeTicker(t *testing.T) {
+	// jitterPct=0 reduces to a deterministic ticker. The mean
+	// should be exactly the interval (modulo scheduling noise).
+	const (
+		interval = 20 * time.Millisecond
+		ticks    = 10
+	)
+
+	jt := NewJitteredTicker(interval, 0)
+	defer jt.Stop()
+
+	last := time.Now()
+	for i := 0; i < ticks; i++ {
+		select {
+		case now := <-jt.C:
+			gap := now.Sub(last)
+			last = now
+			// Allow generous slop for CI scheduling.
+			if i > 0 && (gap < interval/2 || gap > interval*3) {
+				t.Errorf("zero-jitter tick %d gap=%v far from interval=%v", i, gap, interval)
+			}
+		case <-time.After(5 * interval):
+			t.Fatalf("zero-jitter tick %d timed out", i)
+		}
+	}
+}
+
+func TestJitteredTicker_NegativeJitter_TreatedAsZero(t *testing.T) {
+	// Defensive: negative jitterPct should not produce
+	// negative-duration timers (which would panic time.NewTimer).
+	jt := NewJitteredTicker(10*time.Millisecond, -0.5)
+	defer jt.Stop()
+
+	// Just confirm at least one tick fires without panic.
+	select {
+	case <-jt.C:
+		// ok
+	case <-time.After(100 * time.Millisecond):
+		t.Errorf("negative-jitter ticker produced no tick within 100ms")
+	}
+}
+
+func TestJitteredTicker_LargeJitter_ClampedBelowOne(t *testing.T) {
+	// Defensive: jitterPct≥1 would otherwise allow next=0 and panic
+	// time.NewTimer. Confirm the ticker still fires.
+	jt := NewJitteredTicker(10*time.Millisecond, 1.5)
+	defer jt.Stop()
+
+	select {
+	case <-jt.C:
+		// ok
+	case <-time.After(100 * time.Millisecond):
+		t.Errorf("over-clamped-jitter ticker produced no tick within 100ms")
+	}
+}

internal/scheduler/scheduler.go

@@ -473,7 +473,7 @@ func (s *Scheduler) Start(ctx context.Context) <-chan struct{} {
 // If an error occurs, it logs the error but continues running.
 // Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) renewalCheckLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.renewalCheckInterval)
+	ticker := NewJitteredTicker(s.renewalCheckInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -522,7 +522,7 @@ func (s *Scheduler) runRenewalCheck(ctx context.Context) {
 // If an error occurs, it logs the error but continues running.
 // Uses atomic.Bool to prevent duplicate execution if the previous job is still running.
 func (s *Scheduler) jobProcessorLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.jobProcessorInterval)
+	ticker := NewJitteredTicker(s.jobProcessorInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -573,7 +573,7 @@ func (s *Scheduler) runJobProcessor(ctx context.Context) {
 // Uses atomic.Bool to prevent duplicate execution if the previous retry sweep
 // is still running.
 func (s *Scheduler) jobRetryLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.jobRetryInterval)
+	ticker := NewJitteredTicker(s.jobRetryInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -628,7 +628,7 @@ func (s *Scheduler) runJobRetry(ctx context.Context) {
 // retry loop then auto-promotes eligible Failed jobs back to Pending. Closes
 // coverage gap I-003. Uses atomic.Bool to prevent duplicate execution.
 func (s *Scheduler) jobTimeoutLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.jobTimeoutInterval)
+	ticker := NewJitteredTicker(s.jobTimeoutInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -706,7 +706,7 @@ func (s *Scheduler) runJobTimeout(ctx context.Context) {
 // If an error occurs, it logs the error but continues running.
 // Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) agentHealthCheckLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.agentHealthCheckInterval)
+	ticker := NewJitteredTicker(s.agentHealthCheckInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -754,7 +754,7 @@ func (s *Scheduler) runAgentHealthCheck(ctx context.Context) {
 // If an error occurs, it logs the error but continues running.
 // Uses atomic.Bool to prevent duplicate execution if the previous process is still running.
 func (s *Scheduler) notificationProcessLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.notificationProcessInterval)
+	ticker := NewJitteredTicker(s.notificationProcessInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -806,7 +806,7 @@ func (s *Scheduler) runNotificationProcess(ctx context.Context) {
 // Uses atomic.Bool to prevent duplicate execution if the previous retry sweep
 // is still running. Mirrors the I-001 jobRetryLoop topology byte-for-byte.
 func (s *Scheduler) notificationRetryLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.notificationRetryInterval)
+	ticker := NewJitteredTicker(s.notificationRetryInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -861,7 +861,7 @@ func (s *Scheduler) runNotificationRetry(ctx context.Context) {
 // no CRL/OCSP needed.
 // Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) shortLivedExpiryCheckLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.shortLivedExpiryCheckInterval)
+	ticker := NewJitteredTicker(s.shortLivedExpiryCheckInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -909,7 +909,7 @@ func (s *Scheduler) runShortLivedExpiryCheck(ctx context.Context) {
 // of configured network targets.
 // Uses atomic.Bool to prevent duplicate execution if the previous scan is still running.
 func (s *Scheduler) networkScanLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.networkScanInterval)
+	ticker := NewJitteredTicker(s.networkScanInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -956,7 +956,7 @@ func (s *Scheduler) runNetworkScan(ctx context.Context) {
 // digestLoop runs every digestInterval and generates/sends certificate digest emails.
 // Uses atomic.Bool to prevent duplicate execution if the previous digest is still running.
 func (s *Scheduler) digestLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.digestInterval)
+	ticker := NewJitteredTicker(s.digestInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Do NOT run immediately on start for digest — wait for the first tick.
@@ -999,7 +999,7 @@ func (s *Scheduler) runDigest(ctx context.Context) {
 // resource-intensive. Wait for the first tick.
 // Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) healthCheckLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.healthCheckInterval)
+	ticker := NewJitteredTicker(s.healthCheckInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Do NOT run immediately on start for health checks — wait for the first tick.
@@ -1041,7 +1041,7 @@ func (s *Scheduler) runHealthCheck(ctx context.Context) {
 // Runs immediately on start, then on each tick. Same idempotency pattern as networkScanLoop.
 // Uses atomic.Bool to prevent duplicate execution if the previous scan is still running.
 func (s *Scheduler) cloudDiscoveryLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.cloudDiscoveryInterval)
+	ticker := NewJitteredTicker(s.cloudDiscoveryInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Run immediately on start (with idempotency guard)
@@ -1121,7 +1121,7 @@ func (s *Scheduler) WaitForCompletion(timeout time.Duration) error {
 //
 // Bundle CRL/OCSP-Responder Phase 3.
 func (s *Scheduler) crlGenerationLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.crlGenerationInterval)
+	ticker := NewJitteredTicker(s.crlGenerationInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	// Do NOT run immediately on start. CRLs are typically valid for
@@ -1171,7 +1171,7 @@ var ErrSchedulerShutdownTimeout = errors.New("scheduler graceful shutdown timeou
 // sync.WaitGroup tracks the in-flight goroutine for graceful shutdown.
 // Phase 5.
 func (s *Scheduler) acmeGCLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.acmeGCInterval)
+	ticker := NewJitteredTicker(s.acmeGCInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	for {
@@ -1212,7 +1212,7 @@ func (s *Scheduler) acmeGCLoop(ctx context.Context) {
 // file: a stuck Postgres can't block the next tick, and concurrent
 // sweeps are skipped not queued.
 func (s *Scheduler) sessionGCLoop(ctx context.Context) {
-	ticker := time.NewTicker(s.sessionGCInterval)
+	ticker := NewJitteredTicker(s.sessionGCInterval, DefaultSchedulerJitter)
 	defer ticker.Stop()
 
 	for {