## Why
Servarr MediaCover assets (movie/series posters and fanart) and app backups currently live on each replica's local `/config`, which is an `emptyDir`. A poster the leader downloads is invisible to the other replicas behind the Service, so covers render inconsistently and backups are leader-local. This provisions a shared Ceph RGW (S3) bucket to hold those assets instead.
## Changes
- Add an `arrstack-media` `ObjectStoreUser` + `Bucket` (cephrgw-operator), mirroring the CNPG backup pattern. The operator mints the `arrstack-media-s3` credential Secret in-namespace (`AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY`, `S3_ENDPOINT`), 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 at `/etc/ssl/vault-ca/ca.crt` for RGW TLS validation).
## Notes
- The env keys are unknown to the current `-unkin2` image and are ignored by it; they activate once the `-unkin3+` image (radarr MediaCover-S3 support, separate radarr PR) ships. sonarr/prowlarr wiring and backups-to-S3 follow in later PRs.
- `kubectl kustomize apps/base/arrstack` builds clean.
Reviewed-on: #387
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
