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/pvc-mediastore.yaml

@@ -0,0 +1,22 @@
+---
+# Whole media tree (/fafflix, /cheeztv, /nzbget) on one RWX filesystem, shared
+# across the sonarr/radarr/nzbget pods. Statically bound to the
+# arrstack-mediastore PV (the same CephFS subvolume fafflix and cheeztv mount).
+# storageClassName "" + volumeName disables dynamic provisioning and binds the
+# pre-created static PV.
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: mediastore
+  namespace: arrstack
+  annotations:
+    k8up.io/backup: "false"
+spec:
+  accessModes:
+    - ReadWriteMany
+  resources:
+    requests:
+      storage: 10Ti
+  storageClassName: ""
+  volumeName: arrstack-mediastore
+  volumeMode: Filesystem