arrstack,fafflix,cheeztv: provision shared mediastore CephFS volume

Adds the static PV/PVC trio for the new 10Ti "mediastore" CephFS subvolume
plus a bootstrap Job that seeds its directory tree. Purely additive: the
existing media-tv/media-movies volumes are untouched.

- add static PVs arrstack-mediastore, fafflix-mediastore, cheeztv-mediastore
  all pinned to the same rootPath on cephfs_csi_ssd_ec_4_1
- add the matching RWX 10Ti PVCs, k8up-excluded, statically bound by
  volumeName + storageClassName ""
- add mediastore-bootstrap, an ArgoCD Sync-hook Job that mkdir -p's
  /{fafflix,cheeztv}/{tvseries,movies} and /nzbget/downloads/complete as
  uid/gid 1000
- wire the new manifests into the arrstack, fafflix and cheeztv bases

apps/base/arrstack/kustomization.yaml

@@ -8,9 +8,12 @@ resources:
   - pv-media-tv.yaml
   - pv-media-movies.yaml
   - pv-mediafs.yaml
+  - pv-mediastore.yaml
   - pvc-media-tv.yaml
   - pvc-media-movies.yaml
   - pvc-mediafs.yaml
+  - pvc-mediastore.yaml
+  - mediastore-bootstrap-job.yaml
   - media-bucket.yaml
   - backups-bucket.yaml
   - postgres