Add shared arrstack Valkey and wire arr apps' Redis features

Activate the arr forks' #14 Redis features (SignalR backplane, cross-replica
cache-invalidation bus, distributed rate limiter), which ship deployed but inert
because no Valkey exists and nothing is wired.

- Add a single shared ValkeyCluster (arrstack-valkey) under apps/base/arrstack/valkey,
  modeled on jellyfin-valkey: shards:1/replicas:2 HA, image via artifactapi,
  passwordless default user, node spread + cluster PDB, ephemeral storage.
- Register the valkey component in the arrstack base kustomization.
- Wire <App>__Redis__Host/Port into the sonarr/radarr/prowlarr env ConfigMaps,
  all pointing at the shared service valkey-arrstack-valkey:6379. Host is the
  activation switch (RedisOptions.IsConfigured gates on Host; no Enabled flag).
  Passwordless (jellyfin parity) so no Password/Ssl. Per-app key/channel prefixes
  keep the three apps isolated on one cluster.

apps/base/arrstack/kustomization.yaml

@@ -14,6 +14,7 @@ resources:
   - media-bucket.yaml
   - backups-bucket.yaml
   - postgres
+  - valkey
   - sonarr
   - radarr
   - prowlarr