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/valkey/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - valkeycluster.yaml

apps/base/arrstack/valkey/valkeycluster.yaml

@@ -0,0 +1,47 @@
+---
+# Single shared HA Valkey for the arr apps (sonarr/radarr/prowlarr), managed by
+# valkey-operator. It activates the fork's #14 Redis features: the SignalR
+# backplane, the cross-replica cache-invalidation bus, and the distributed rate
+# limiter. One cluster is safe for all three because each fork namespaces its
+# keys and pub/sub channels by a per-app prefix (sonarr:ratelimit: /
+# radarr:ratelimit: / prowlarr:ratelimit:), so their state never collides.
+#
+# Modeled on jellyfin-valkey: shards:1 + replicas:2 is one primary with two
+# replicas in a single shard group (three ValkeyNodes total); losing the primary
+# triggers an automatic failover so a node/pod loss no longer drops the shared
+# state the app replicas coordinate through. The operator runs Valkey
+# cluster-mode-enabled with protected-mode off and leaves the built-in `default`
+# user passwordless, so clients connect with no auth/TLS; StackExchange.Redis
+# seeds off the single service and auto-discovers topology plus failovers.
+# scheduling.node.spread.shard:Required keeps the three nodes on distinct hosts,
+# so one host loss removes at most one node; podDisruptionBudget.mode:Cluster
+# lets the operator manage a quorum-aware PDB. Persistence is omitted (/data is an
+# emptyDir): the coordination state is ephemeral (short TTLs / transient pub/sub),
+# replication+failover already provide redundancy, and an operator-managed PVC
+# cannot carry the k8up.io/backup:"false" annotation the namespace k8up Schedule
+# needs to skip in-use RWO volumes.
+apiVersion: valkey.io/v1alpha1
+kind: ValkeyCluster
+metadata:
+  name: arrstack-valkey
+  namespace: arrstack
+spec:
+  shards: 1
+  replicas: 2
+  image: artifactapi.k8s.syd1.au.unkin.net/dockerhub/valkey/valkey:9.0.0
+  exporter:
+    enabled: false
+  scheduling:
+    node:
+      spread:
+        shard:
+          mode: Required
+  podDisruptionBudget:
+    mode: Cluster
+  resources:
+    requests:
+      cpu: 50m
+      memory: 128Mi
+    limits:
+      cpu: 500m
+      memory: 512Mi