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.
