[redis] Phase 7 step 2: remove Bitnami helm_release + orphan PVCs

Bringing the 2026-04-19 rework to its end-state. Cutover soaked for ~1h
with 0 alerts firing and 127 ops/sec on the v2 master — skipped the
nominal 24h rollback window per user direction.

 - Removed `helm_release.redis` (Bitnami chart v25.3.2) from TF. Helm
   destroy cleaned up the StatefulSet redis-node (already scaled to 0),
   ConfigMaps, ServiceAccount, RBAC, and the deprecated `redis` + `redis-headless`
   ClusterIP services that the chart owned.
 - Removed `null_resource.patch_redis_service` — the kubectl-patch hack
   that worked around the Bitnami chart's broken service selector. No
   Helm chart, no patch needed.
 - Removed the dead `depends_on = [helm_release.redis]` from the HAProxy
   deployment.
 - `kubectl delete pvc -n redis redis-data-redis-node-{0,1}` for the two
   orphan PVCs the StatefulSet template left behind (K8s doesn't cascade-delete).
 - Simplified the top-of-file comment and the redis-v2 architecture
   comment — they talked about the parallel-cluster migration state that
   no longer exists. Folded in the sentinel hostname gotcha, the redis
   8.x image requirement, and the BGSAVE+AOF-rewrite memory reasoning
   so the rationale survives in the code rather than only in beads.
 - `RedisDown` alert no longer matches `redis-node|redis-v2` — just
   `redis-v2` since that's the only StatefulSet now. Kept the `or on()
   vector(0)` so the alert fires when kube_state_metrics has no sample
   (e.g. after accidental delete).
 - `docs/architecture/databases.md` trimmed: no more "pending TF removal"
   or "cold rollback for 24h" language.

Verification after apply:
 - kubectl get all -n redis: redis-v2-{0,1,2} (3/3 Running) + redis-haproxy-*
   (3 pods, PDB minAvailable=2). Services: redis-master + redis-v2-headless only.
 - PVCs: data-redis-v2-{0,1,2} only (redis-data-redis-node-* deleted).
 - Sentinel: all 3 agree mymaster = redis-v2-0 hostname.
 - HAProxy: PING PONG, DBSIZE 92, 127 ops/sec on master.
 - Prometheus: 0 firing redis alerts.

Closes: code-v2b
Closes: code-2mw

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stacks/monitoring/modules/monitoring/prometheus_chart_values.tpl

@@ -1355,15 +1355,12 @@ serverFiles:
             annotations:
               summary: "PostgreSQL pod {{ $labels.pod }} is not ready"
           - alert: RedisDown
-            # Covers both the legacy Bitnami StatefulSet (redis-node) and the
-            # new raw StatefulSet (redis-v2) during the 2026-04-19 migration.
-            # Drop the redis-node branch after helm_release.redis is removed.
-            expr: (sum(kube_statefulset_status_replicas_ready{namespace="redis", statefulset=~"redis-node|redis-v2"}) or on() vector(0)) < 1
+            expr: (kube_statefulset_status_replicas_ready{namespace="redis", statefulset="redis-v2"} or on() vector(0)) < 1
             for: 5m
             labels:
               severity: critical
             annotations:
-              summary: "Redis has no ready replicas across both clusters"
+              summary: "Redis has no ready replicas"
           - alert: RedisMemoryPressure
             expr: redis_memory_used_bytes{namespace="redis"} / redis_memory_max_bytes{namespace="redis"} > 0.85
             for: 5m