[redis] Phase 3-7: cutover to redis-v2, Nextcloud HAProxy-only

Phase 3 — replication chain (old → v2):
 - Discovered the v2 cluster was running redis:7.4-alpine, but the
   Bitnami old master ships redis 8.6.2 which writes RDB format 13 —
   the 7.4 replicas rejected the stream with "Can't handle RDB format
   version 13". Bumped v2 image to redis:8-alpine (also 8.6.2) to
   restore PSYNC compatibility.
 - Discovered that sentinel on BOTH v2 and old Bitnami clusters
   auto-discovered the cross-cluster replication chain when v2-0
   REPLICAOF'd the old master, triggering a failover that reparented
   old-master to a v2 replica and took HAProxy's backend offline.
   Mitigation: `SENTINEL REMOVE mymaster` on all 5 sentinels (both
   clusters) during the REPLICAOF surgery, then re-MONITOR after
   cutover. This must be done on the OLD sentinels too, not just v2 —
   they're the ones that kept fighting our REPLICAOF.
 - Set up the chain: v2-0 REPLICAOF old-master; v2-{1,2} REPLICAOF v2-0.
   All 76 keys (db0:76, db1:22, db4:16) synced including `immich_bull:*`
   BullMQ queues and `_kombu.*` Celery queues — the user-stated
   must-survive data class.

Phase 4 — HAProxy cutover:
 - Updated `kubernetes_config_map.haproxy` to point at
   `redis-v2-{0,1,2}.redis-v2-headless` for both redis_master and
   redis_sentinel backends (removed redis-node-{0,1}).
 - Promoted v2-0 (`REPLICAOF NO ONE`) at the same time as the
   ConfigMap apply so HAProxy's 1s health-check interval found a
   role:master within a few seconds. Cutover disruption on HAProxy
   rollout was brief; old clients naturally moved to new HAProxy pods
   within the rolling update window.
 - Re-enabled sentinel monitoring on v2 with `SENTINEL MONITOR
   mymaster <hostname> 6379 2` after verifying `resolve-hostnames yes`
   + `announce-hostnames yes` were active — this ensures sentinel
   stores the hostname (not resolved IP) in its rewritten config, so
   pod-IP churn on restart doesn't break failover.

Phase 5 — chaos:
 - Round 1: killed master v2-0 mid-probe. First run exposed the
   sentinel IP-storage issue (stored 10.10.107.222, went stale on
   restart) — ~12s probe disruption. Fixed hostname persistence and
   re-MONITORed.
 - Round 2: killed new master v2-2 with hostnames correctly stored.
   Sentinel elected v2-0, HAProxy re-routed, 1/40 probe failures over
   60s — target <3s of actual user-visible disruption.

Phase 6 — Nextcloud simplification:
 - `zzz-redis.config.php` no longer queries sentinel in-process —
   just points at `redis-master.redis.svc.cluster.local`. Removed 20
   lines of PHP. HAProxy handles master tracking transparently now
   that it's scaled to 3 + PDB minAvailable=2.

Phase 7 step 1:
 - `kubectl scale statefulset/redis-node --replicas=0` (transient —
   TF removal in a 24h follow-up). Old PVCs `redis-data-redis-node-{0,1}`
   preserved as cold rollback.

Docs:
 - Rewrote `databases.md` Redis section to reflect post-cutover reality
   and the sentinel hostname gotcha (so future sessions don't relearn it).
 - `.claude/reference/service-catalog.md` entry updated.

The parallel-bootstrap race documented in the previous commit is still
worth watching — the init container now defaults to pod-0 as master
when no peer reports role:master-with-slaves, so fresh boots land in
a deterministic topology.

Closes: code-7n4
Closes: code-9y6
Closes: code-cnf
Closes: code-tc4

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

stacks/nextcloud/chart_values.yaml

@@ -29,35 +29,19 @@ nextcloud:
   configs:
     zzz-redis.config.php: |
       <?php
-      // Redis with Sentinel-based master discovery
-      // Queries Sentinel to find the current master, falls back to HAProxy service
-      // which health-checks Redis nodes and routes only to the master.
-      $sentinels = [
-        ['redis-node-0.redis-headless.redis.svc.cluster.local', 26379],
-        ['redis-node-1.redis-headless.redis.svc.cluster.local', 26379],
-      ];
-      // Fallback: HAProxy master-only service (safe even if Sentinel is unavailable)
-      $redisHost = 'redis-master.redis.svc.cluster.local';
-      $redisPort = 6379;
-      foreach ($sentinels as [$sHost, $sPort]) {
-        try {
-          $s = new Redis();
-          if ($s->connect($sHost, $sPort, 0.5)) {
-            $master = $s->rawCommand('SENTINEL', 'get-master-addr-by-name', 'mymaster');
-            if ($master) {
-              $redisHost = $master[0];
-              $redisPort = (int)$master[1];
-              break;
-            }
-          }
-        } catch (\Exception $e) {}
-      }
+      // Redis via HAProxy master-only service. HAProxy (3 replicas, PDB
+      // minAvailable=2) health-checks all v2 pods via `INFO replication` and
+      // routes to the current role:master. Sentinel failover takes <30s, and
+      // HAProxy detects the new master via its 1s tcp-check interval and
+      // starts routing within ~3s of detection. Removed the old in-process
+      // sentinel-query loop on 2026-04-19 after the Redis rework — see
+      // beads code-v2b and infra/docs/architecture/databases.md.
       $CONFIG = array(
         'memcache.distributed' => '\\OC\\Memcache\\Redis',
         'memcache.locking' => '\\OC\\Memcache\\Redis',
         'redis' => array(
-          'host' => $redisHost,
-          'port' => $redisPort,
+          'host' => 'redis-master.redis.svc.cluster.local',
+          'port' => 6379,
           'password' => '',
           'timeout' => 1.5,
           'read_timeout' => 1.5,