docs(kms): add rotation driver matrix and rotation-overdue alert (#5992)

Add a per-backend rotation-driver matrix to docs/operations/kms-backend-security.md: who performs the rotation on each backend (RustFS for Vault KV2, Vault's Transit engine for Transit, AWS RotateKeyOnDemand for AWS, nobody for Local/Static), how periodic rotation must be scheduled on each (external scheduler for KV2 by design, Vault auto_rotate_period for Transit, AWS-native automatic rotation for AWS since RotateKeyOnDemand carries a lifetime quota), the NIST SP 800-38D 2^32 random-nonce AES-GCM wrap ceiling that Local/Static can never reset, and a pre-rotation checklist referencing the existing upgrade-ordering hard constraint.

Add the KmsKeyRotationOverdue Prometheus rule on rustfs_kms_oldest_key_rotation_age_seconds (400-day conservative default, warning severity, no traffic guard because it is direct gauge state) and its runbook response procedure in docs/operations/kms-observability-runbook.md, following the existing per-alert format.

Sharpen the runbook's rotation-timestamp paragraph with verified per-backend behavior: only Vault KV2 persists rotated_at (stamped in the same check-and-set write that commits the rotation), while Transit and AWS key listings always report it absent, so on those backends the gauge measures key age and does not reset on rotation. Update Threshold calibration and Coverage gaps for the new rule.

Validated with promtool check rules (7 rules, SUCCESS) and scripts/check_doc_paths.sh.

Part of rustfs/backlog#1636 (PR-4).

.docker/observability/prometheus-rules/rustfs-kms-alerts.yml

@@ -17,9 +17,11 @@
 # =============================================================================
 #
 # Metric source: the KMS operation-policy choke point in
-# crates/kms/src/policy.rs. All label values are bounded static strings
-# (operation, op_class, outcome, error_class, backend, scope); key identifiers,
-# key material, and tokens never appear in labels.
+# crates/kms/src/policy.rs, except KmsKeyRotationOverdue, which reads the
+# label-less key-lifecycle gauge published by the deletion worker's sweep
+# (crates/kms/src/deletion_worker.rs). All label values are bounded static
+# strings (operation, op_class, outcome, error_class, backend, scope); key
+# identifiers, key material, and tokens never appear in labels.
 #
 # Response procedures: docs/operations/kms-observability-runbook.md
 #
@@ -212,3 +214,38 @@ groups:
             circuit_open until the half-open probe succeeds or returns
             a non-retryable failure.
           runbook_url: "https://github.com/rustfs/rustfs/blob/main/docs/operations/kms-observability-runbook.md#kmsbackendcircuitopen"
+
+      # ------------------------------------------------------------------
+      # 7. KmsKeyRotationOverdue
+      #    The least recently rotated usable key has gone more than 400
+      #    days without a rotation (measured from creation for keys with
+      #    no recorded rotation). Direct gauge state published by the
+      #    deletion worker's sweep, so no traffic guard applies; the
+      #    one-hour hold only bridges scrape gaps. The worker runs only
+      #    on backends with the schedule_deletion capability, so on the
+      #    Static backend the series never exists and this alert cannot
+      #    fire — that backend cannot rotate either; see the rotation
+      #    driver matrix in docs/operations/kms-backend-security.md.
+      #    Threshold: 400 days — conservative default sitting above a
+      #    one-year rotation policy. Align it with the rotation period
+      #    your compliance policy requires, and with
+      #    RUSTFS_KMS_ROTATION_MAX_AGE_SECS so the per-key rotation_due
+      #    verdict and this aggregate alert agree.
+      # ------------------------------------------------------------------
+      - alert: KmsKeyRotationOverdue
+        expr: |
+          rustfs_kms_oldest_key_rotation_age_seconds > (400 * 86400)
+        for: 1h
+        labels:
+          severity: warning
+          component: kms
+        annotations:
+          summary: "Oldest KMS key unrotated for more than 400 days"
+          description: >-
+            The least recently rotated usable KMS key was last rotated
+            {{ $value | humanizeDuration }} ago (measured from creation
+            for keys with no recorded rotation). List keys through the
+            admin API and read rotation_due / rotation_due_reason for
+            the per-key verdict; an "unsupported" reason means the
+            backend cannot rotate at all.
+          runbook_url: "https://github.com/rustfs/rustfs/blob/main/docs/operations/kms-observability-runbook.md#kmskeyrotationoverdue"