arrstack: route app backups to S3 and wire sonarr MediaCover to the shared bucket

## Why

Servarr backups (the periodic config+DB zips) and sonarr MediaCover posters
still live on each replica's local /config, which is an emptyDir. Backups are
leader-local and lost when the pod is rescheduled; sonarr covers render
inconsistently across the 3 active-active replicas. #387 already moved radarr
covers to a shared Ceph RGW bucket; this extends the same pattern to backups for
all three apps and to sonarr's covers.

## Changes

- Add an `arrstack-backups` ObjectStoreUser + Bucket (cephrgw-operator), minting
  the `arrstack-backups-s3` credential Secret in-namespace (no Vault KV seeding).
  Backups get their own bucket (not arrstack-media) because they carry secrets
  (Config.xml API keys, the DB) and warrant separate credentials/lifecycle from
  poster art; the three apps share it, isolated by a per-app key prefix.
- Wire sonarr to the shared `arrstack-media` bucket via `Sonarr__MediaCoverS3__*`
  (prefix `sonarr`, partitioned from radarr's `radarr` prefix), plus the estate
  CA mount at /etc/ssl/vault-ca/ca.crt for RGW TLS validation.
- Wire radarr/sonarr/prowlarr backups via `<App>__BackupS3__*` to
  `arrstack-backups` (per-app prefix); add the CA mount to sonarr and prowlarr.

## Notes

- The env keys are consumed by the -unkin3+ images (separate app PRs) and
  ignored by older images, so this is safe to apply ahead of the image bump.
- `kubectl kustomize apps/base/arrstack` builds clean.

apps/base/arrstack/sonarr/deployment.yaml

@@ -98,6 +98,61 @@ spec:
                 secretKeyRef:
                   name: sonarr-apikey
                   key: apitoken
+            # MediaCover object store (shared arrstack-media Ceph RGW bucket,
+            # partitioned from radarr by the sonarr key prefix). Serves
+            # posters/fanart from S3 so any replica renders them instead of the
+            # leader-local emptyDir /config. Consumed by the -unkin3+ image;
+            # older images ignore these unknown config keys. Creds Secret minted
+            # by cephrgw-operator.
+            - name: Sonarr__MediaCoverS3__Endpoint
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-media-s3
+                  key: S3_ENDPOINT
+            - name: Sonarr__MediaCoverS3__AccessKey
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-media-s3
+                  key: AWS_ACCESS_KEY_ID
+            - name: Sonarr__MediaCoverS3__SecretKey
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-media-s3
+                  key: AWS_SECRET_ACCESS_KEY
+            - name: Sonarr__MediaCoverS3__Bucket
+              value: arrstack-media
+            - name: Sonarr__MediaCoverS3__Prefix
+              value: sonarr
+            - name: Sonarr__MediaCoverS3__ForcePathStyle
+              value: "true"
+            - name: Sonarr__MediaCoverS3__CaCertPath
+              value: /etc/ssl/vault-ca/ca.crt
+            # Backup object store (shared arrstack-backups Ceph RGW bucket,
+            # per-app key prefix). Routes the periodic config+DB zip backups off
+            # the ephemeral /config so any replica can write and restore them.
+            - name: Sonarr__BackupS3__Endpoint
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-backups-s3
+                  key: S3_ENDPOINT
+            - name: Sonarr__BackupS3__AccessKey
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-backups-s3
+                  key: AWS_ACCESS_KEY_ID
+            - name: Sonarr__BackupS3__SecretKey
+              valueFrom:
+                secretKeyRef:
+                  name: arrstack-backups-s3
+                  key: AWS_SECRET_ACCESS_KEY
+            - name: Sonarr__BackupS3__Bucket
+              value: arrstack-backups
+            - name: Sonarr__BackupS3__Prefix
+              value: sonarr
+            - name: Sonarr__BackupS3__ForcePathStyle
+              value: "true"
+            - name: Sonarr__BackupS3__CaCertPath
+              value: /etc/ssl/vault-ca/ca.crt
           livenessProbe:
             httpGet:
               path: /sonarr/ping
@@ -126,9 +181,19 @@ spec:
               mountPath: /config
             - name: media-tv
               mountPath: /media/tv
+            - name: vault-ca
+              mountPath: /etc/ssl/vault-ca
+              readOnly: true
       volumes:
         - name: config
           emptyDir: {}
         - name: media-tv
           persistentVolumeClaim:
             claimName: media-tv
+        # Estate CA for validating the Ceph RGW (s3.ceph.unkin.net) TLS cert.
+        - name: vault-ca
+          secret:
+            secretName: vault-ca-cert
+            items:
+              - key: ca.crt
+                path: ca.crt