chore(storage): Longhorn over-provisioning 100->200% + right-size plex config 20->8Gi

The 39Gi/node Longhorn data disks at 1:1 over-provisioning can't fit the media stack
(plex 20Gi replica failed: 'insufficient storage'). Volumes are thin-provisioned config-only,
so 200% lets the full catalog fit without a disk resize; physical fill risk is low. Also
right-sized plex config (20Gi was excessive for metadata). Prod-grade path: grow the data
disks (WS-STORAGE follow-up).

apps/plex.yaml

@@ -49,7 +49,7 @@ spec:
   storageClassName: longhorn
   resources:
     requests:
-      storage: 20Gi
+      storage: 8Gi
 ---
 # Deployment (R-PLAT-02)
 apiVersion: apps/v1