M-2 PR-F: Middleware/ACME ctx-propagation + contextcheck linter + audit closeout

Final PR in the six-commit M-2 sequence (PR-A: CertificateService cluster
cdc9d03, PR-B: IssuerService+TargetService eb14236, PR-C: Policy/Profile/
Owner/Team 2497be4, PR-D: Job/Notification/Audit ccd89c3, PR-E: AgentService
283ec27, PR-F: this commit). PR-A through PR-E collapsed the service-layer
shim methods and deleted every in-production context.Background() /
context.TODO() call from internal/service/; this PR completes the sweep
across the non-service tiers (HTTP middleware + ACME connector) and wires
the contextcheck linter so regressions fail CI.

Three narrow edits land the D-3 pattern (context.WithoutCancel for
subsidiary async writes and deferred shutdown contexts):

  - internal/api/middleware/audit.go  -- async audit goroutine now runs
    on auditCtx := context.WithoutCancel(r.Context()) instead of
    context.Background(). Preserves request-scoped values (trace ID, auth)
    while detaching from the request's cancellation so the audit write
    does not get killed when the response completes. Goroutine is still
    tracked via a.wg (M-1 shutdown drain) so Flush(ctx) behaviour is
    unchanged. CWE-770 Missing Release (goroutine leak potential) +
    CWE-400 Resource Exhaustion (missed cancellation propagation).

  - internal/api/middleware/middleware.go -- Recovery panic path now
    logs via slog.ErrorContext(ctx, ...) instead of log.Printf. Request-
    scoped trace/auth metadata now carries through the panic log, matching
    every other request log. D-3 non-bypass: the context is r.Context()
    captured before the defer, so even a panic mid-handler propagates
    the ctx's trace ID into the ERROR log line.

  - internal/connector/issuer/acme/acme.go (HTTP-01 challenge server
    shutdown) -- defer shutdown context derived from
    context.WithTimeout(context.WithoutCancel(ctx), 5s) instead of
    context.Background(). Preserves parent ctx values, detaches from
    parent cancellation so Shutdown always gets its full 5-second
    budget even when the parent was cancelled. Matches the same pattern
    applied in ACME's solveAuthorizationsDNS01 and solveAuthorizationsDNSPersist01.

Linter wiring: .golangci.yml adds `contextcheck` to the enabled set.
golangci-lint v2.11.4 now fails CI on any function that takes a
context.Context parameter but calls into context.Background() or
context.TODO() instead of propagating -- regression guard for all five
prior PRs.

Verification (CI parity, GOCACHE=/tmp/gocache GOMODCACHE=/tmp/gomodcache
GOLANGCI_LINT_CACHE=/tmp/lintcache):

  - go build ./... -> 0
  - go vet ./... -> 0
  - golangci-lint run (contextcheck enabled) -> 0 issues
  - go test -race -short ./internal/api/middleware/... -> PASS
  - go test -race -short ./internal/scheduler/... -> PASS
  - go test -race -short ./internal/connector/issuer/acme/... -> PASS
  - go test -race -short ./internal/service/... -> PASS
  - rg "context\.(Background|TODO)\(\)" internal/service/ internal/scheduler/
    internal/connector/ internal/api/middleware/ -> 0 non-test hits
    (one pedagogical godoc reference in audit.go documenting why
    context.Background() would be wrong remains intentional)

Wire-format invariants preserved: 0 API routes, 0 SQL migrations, 0
frontend bytes, 0 OpenAPI bytes, 0 connector interface signature changes,
0 new env vars, 0 new external dependencies (pure context stdlib). The
AuditRecorder interface signature, the body-hash algorithm (SHA-256 16
hex chars), the excluded-path short-circuit, the actor-extraction path,
the responseWriter status-capture wrapper, the AuditServiceAdapter, and
all 116 API routes under /api/v1/, /.well-known/est/, /scep, /health,
/auth are byte-identical.

M-2 aggregate across PR-A through PR-F: 57 files, +635 / -613 (PR-A 12f
+227/-237, PR-B 9f +150/-146, PR-C 17f +156/-148, PR-D 11f +67/-63,
PR-E 4f +9/-15, PR-F 4f +26/-4). With M-2 closed, 8 of 10 Medium
findings resolved; M-9, M-10, L-1..L-4, I-1..I-8 remain post-v2.1.0
hardening batch.

Audit complete. Commit: 1f6cf0eafa. Sections: 12. Findings: 2/7/10/4/6.

internal/api/middleware/audit.go

@@ -132,6 +132,15 @@ func (a *AuditMiddleware) Middleware(next http.Handler) http.Handler {
 		path := r.URL.Path
 		status := wrapped.statusCode
 
+		// Derive a detached context that preserves request-scoped values
+		// (trace IDs, auth info carried via context keys) but is not cancelled
+		// when the HTTP server finalizes the request. Using r.Context()
+		// directly would cause the async audit write to observe ctx.Done()
+		// as soon as the response completes; using context.Background() would
+		// discard useful observability metadata. WithoutCancel gives us both
+		// (M-2 / D-3).
+		auditCtx := context.WithoutCancel(r.Context())
+
 		// Record audit event asynchronously (best-effort, don't block response).
 		// SECURITY: We intentionally use r.URL.Path (not r.URL.String() or r.RequestURI)
 		// to prevent query parameters from being recorded in the immutable audit trail.
@@ -147,7 +156,7 @@ func (a *AuditMiddleware) Middleware(next http.Handler) http.Handler {
 		go func() {
 			defer a.wg.Done()
 			if err := a.recorder.RecordAPICall(
-				context.Background(),
+				auditCtx,
 				method,
 				path,
 				actor,