arrstack: provision shared Ceph RGW bucket for media assets

Servarr MediaCover (posters/fanart) and backups currently live on each
replica's local /config, which is an emptyDir - so covers a leader downloads
are invisible to the other replicas behind the Service. Provision a shared S3
bucket to hold these assets instead.

- Add an arrstack-media ObjectStoreUser + Bucket (cephrgw-operator), mirroring
  the CNPG backup pattern; the operator mints the arrstack-media-s3 credential
  Secret in-namespace, so no Vault KV seeding is required.
- Wire the radarr Deployment to the bucket via Radarr__MediaCover__S3__* env
  (creds from the operator Secret, estate CA mounted for RGW TLS). Unknown to
  the current image and activated by the -unkin3+ MediaCover-S3 build; sonarr
  and prowlarr wiring follow.

apps/base/arrstack/kustomization.yaml

@@ -9,6 +9,7 @@ resources:
   - pv-media-movies.yaml
   - pvc-media-tv.yaml
   - pvc-media-movies.yaml
+  - media-bucket.yaml
   - postgres
   - sonarr
   - radarr