notifications: per-policy multi-channel expiry-alert routing

Closes Rank 4 of the 2026-05-03 Infisical deep-research deliverable
(see cowork/infisical-deep-research-results.md Part 5). Pre-fix,
RenewalService.CheckExpiringCertificates already ran daily,
RenewalPolicy.AlertThresholdsDays drove per-cert thresholds, and
NotificationService.SendThresholdAlert deduped per (cert, threshold)
— but the channel was hardcoded to Email
(internal/service/notification.go:118 pre-fix). Operators who
configured PagerDuty / Slack / Teams / OpsGenie via
CERTCTL_PAGERDUTY_ROUTING_KEY etc. got nothing at any threshold
unless SMTP was also wired. Their first signal of an expired cert
was a 3 AM outage.

This commit lands the routing matrix on top of the existing
infrastructure:

  1. RenewalPolicy gains AlertChannels (per-tier channel list) +
     AlertSeverityMap (per-threshold tier assignment) +
     EffectiveAlertChannels / EffectiveAlertSeverity accessors.
     Default*() helpers preserve the back-compat Email-only
     behaviour for operators who haven't touched their policies
     post-upgrade. Migration 000026 adds the JSONB columns
     idempotently.
  2. NotificationService.SendThresholdAlertOnChannel — the new
     per-channel dispatch helper. Old SendThresholdAlert stays as
     an Email-only alias so non-policy callers (admin "send test
     alert" surfaces) keep working byte-for-byte.
  3. NotificationService.HasThresholdNotificationOnChannel — per-
     (cert, threshold, channel) deduplication so a transient
     PagerDuty 5xx today does NOT suppress today's Slack alert and
     tomorrow's PagerDuty retry will still fire.
  4. RenewalService.sendThresholdAlerts walks the resolved channel
     set per threshold tier, fans out to every configured channel,
     handles per-channel failures independently, defensively drops
     off-enum channels with an audit row trail, and records a per-
     channel audit event with metadata.channel + metadata.severity_tier.
  5. service.ExpiryAlertMetrics — atomic counter table mirrored on
     the VaultRenewalMetrics shape from the 2026-05-03 audit fix #5
     (commit 0792271). Three labels: channel × threshold × result
     (success / failure / deduped). Cardinality bound: 6 × 4 × 3 =
     72 series for the standard 4-threshold matrix.
  6. handler.MetricsHandler.SetExpiryAlerts wires the Prometheus
     exposer for certctl_expiry_alerts_total{channel,threshold,result}.
     Pre-sorted snapshot for byte-stable emission.
  7. cmd/server/main.go threads ONE service.ExpiryAlertMetrics
     instance through both the recording side (notificationService.
     SetExpiryAlertMetrics) and the exposing side
     (metricsHandler.SetExpiryAlerts).

Dispatch flow (post-fix, per renewal-loop tick):

  cert ages past T-30  → daily renewal-loop fires
                       → policy lookup
                       → for each crossed threshold:
                           - resolve severity tier (informational/
                             warning/critical) via AlertSeverityMap
                           - look up channel set in AlertChannels[tier]
                           - for each channel: dedup → SendThresholdAlertOnChannel
                             → notifierRegistry[channel] → audit row →
                             Prometheus counter increment

Tests (internal/service/renewal_expiry_alerts_test.go):

  TestExpiryAlerts_DefaultMatrix_EmailOnly
  TestExpiryAlerts_PerTierFanOut
  TestExpiryAlerts_PerChannelDedup
  TestExpiryAlerts_OneChannelFails_OthersStillFire
  TestExpiryAlerts_OffEnumChannelDropped
  TestExpiryAlerts_MetricCounterIncrements
  TestExpiryAlerts_NilPolicy_FallsToDefault
  TestExpiryAlerts_OperatorOptOutOfTier

The PerTierFanOut test wires 6 mock notifiers, drives a cert at 0
days through the canonical 4 thresholds with the matrix
{informational:[Slack], warning:[Slack,Email],
critical:[PagerDuty,OpsGenie,Email]}, and asserts the exact
recipient counts: Slack=3, Email=3, PagerDuty=1, OpsGenie=1, no
Teams, no Webhook. The OneChannelFails test pins that PagerDuty
returning a 503 does NOT skip Slack/Email at the same threshold.

Drive-by fix (internal/service/testutil_test.go): the existing
mockNotifRepo.List ignored its filter and returned all rows, which
let legacy tests pass on dedup-via-substring even though the
postgres repo actually applied the filter. Updated the mock to
honour CertificateID / Type / Status / Channel / MessageLike
filters in the same shape as the postgres implementation
(internal/repository/postgres/notification.go). All pre-existing
service tests still pass — the legacy test suite happened to be
robust to the mock filter doing nothing.

Documentation:
  - docs/connectors.md Notifier section gains "Routing expiry
    alerts across channels" — operator-facing, JSON example,
    procurement playbook ("How do I make sure PagerDuty pages on
    the T-1 alert?"), debug recipe via SQL on audit_events +
    notification_events + Prometheus.
  - docs/runbook-expiry-alerts.md — sysadmin-grade flowchart,
    per-policy channel-matrix configuration recipes, "did the on-
    call team get paged?" SQL queries, cardinality budget, V3-Pro
    forward path.
  - cowork/WORKSPACE-ROADMAP.md gains "Multi-channel expiry
    alerts: per-owner routing" V3-Pro entry under Adapter
    hardening.

Out of scope (intentional, flagged in V3-Pro forward path):
  - Per-owner / per-team / per-tenant channel routing (matrix is
    per-policy today, not per-owner).
  - Calendar-aware suppression (no T-30 alerts on weekends).
  - Escalation chains (T-1 unanswered for 30m → escalate).
  - Per-channel rate limiting (downstream of I-005 retry+DLQ).

CHANGELOG.md is intentionally not hand-edited per CHANGELOG.md
itself ("no longer maintains a hand-edited per-version changelog;
per-release notes are auto-generated from commit messages between
consecutive tags").

Verified locally:
- gofmt clean.
- go vet ./internal/domain/... ./internal/service/...
  ./internal/api/handler/... ./cmd/server/...  clean.
  (./internal/repository/postgres/... vet failed on transitive
  testcontainers/docker module download — sandbox disk pressure,
  not a code issue; postgres-repo build succeeds and tests pass.)
- go test -short -count=1 ./internal/domain/...
  ./internal/service/... ./internal/api/handler/...  green.
- go test -race -count=10 -run 'TestExpiryAlerts'
  ./internal/service/...  green (per-channel dedup race-free).

Reference: cowork/infisical-deep-research-results.md Part 5 Rank 4.
Acquisition prompt: cowork/rank-4-multichannel-expiry-alerts-prompt.md.

migrations/000026_renewal_policy_channel_matrix.up.sql

@@ -0,0 +1,23 @@
+-- Rank 4 of the 2026-05-03 Infisical deep-research deliverable
+-- (cowork/infisical-deep-research-results.md Part 5). Adds the
+-- per-policy channel matrix that the multi-channel expiry-alert
+-- routing reads from. Two JSONB columns:
+--
+--   alert_channels      — map[severity_tier][]channel_name. Default
+--                         is '{}' so the runtime falls through to
+--                         domain.DefaultAlertChannels() (Email-only
+--                         across all tiers, the back-compat
+--                         behaviour).
+--   alert_severity_map  — map[threshold_days]severity_tier. Default
+--                         is '{}' so the runtime falls through to
+--                         domain.DefaultAlertSeverityMap() (the
+--                         canonical 30/14/7/0 → informational/warning/
+--                         warning/critical mapping).
+--
+-- Both columns use IF NOT EXISTS so the migration is idempotent —
+-- safe to re-run on every certctl-server boot per the
+-- "Idempotent migrations" architecture decision in CLAUDE.md.
+
+ALTER TABLE renewal_policies
+    ADD COLUMN IF NOT EXISTS alert_channels JSONB NOT NULL DEFAULT '{}'::jsonb,
+    ADD COLUMN IF NOT EXISTS alert_severity_map JSONB NOT NULL DEFAULT '{}'::jsonb;