arrstack: deploy -unkin4 (Redis SignalR backplane + S3 storage) (#390)

## Why

The cluster runs the arrstack forks at `-unkin3`. Ben merged two workstreams into each fork's `main`:

- **Redis SignalR backplane + cross-replica cache invalidation** — SignalR messages and cache-invalidation events fan out across replicas via Redis, so any replica serves consistent live UI/state.
- **MediaCover -> S3 + backups -> S3** — MediaCover (sonarr/radarr) and backups (all three) use an optional S3 store, so any stateless replica can serve covers and write/restore backups.

Fresh `-unkin4` tags were cut on each fork's `main` HEAD, building `docker-internal/<app>:<base>-unkin4` images. This bumps the deployment images so ArgoCD rolls the merged work into the cluster.

## Changes

- Bump sonarr app image `v5.0.0-unkin3` -> `v5.0.0-unkin4`
- Bump radarr app image `v6.4.2-unkin3` -> `v6.4.2-unkin4`
- Bump prowlarr app image `v2.6.2-unkin3` -> `v2.6.2-unkin4`

The waitfordb initContainer and bucket resources are unchanged; only the app-container `image:` tags move.

Reviewed-on: #390
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/arrstack/prowlarr/deployment.yaml

@@ -64,7 +64,7 @@ spec:
               memory: 64Mi
       containers:
         - name: prowlarr
-          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/prowlarr:v2.6.2-unkin3
+          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/prowlarr:v2.6.2-unkin4
           imagePullPolicy: IfNotPresent
           command:
             - /app/Prowlarr

apps/base/arrstack/radarr/deployment.yaml

@@ -64,7 +64,7 @@ spec:
               memory: 64Mi
       containers:
         - name: radarr
-          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/radarr:v6.4.2-unkin3
+          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/radarr:v6.4.2-unkin4
           imagePullPolicy: IfNotPresent
           command:
             - /app/Radarr

apps/base/arrstack/sonarr/deployment.yaml

@@ -64,7 +64,7 @@ spec:
               memory: 64Mi
       containers:
         - name: sonarr
-          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/sonarr:v5.0.0-unkin3
+          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/sonarr:v5.0.0-unkin4
           imagePullPolicy: IfNotPresent
           command:
             - /app/Sonarr