fix(media): remove filestash service

- Remove filestash service and volume from media/compose.yaml
- Remove filestash-backup plan from backrest config
- Remove MEDIA_FILESTASH_* environment variables from arty.yml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

core/backrest/config.json

@@ -112,23 +112,6 @@
         }
       }
     },
-    {
-      "id": "filestash-backup",
-      "repo": "hidrive-backup",
-      "paths": [
-        "/volumes/filestash_data"
-      ],
-      "schedule": {
-        "cron": "0 7 * * *"
-      },
-      "retention": {
-        "policyTimeBucketed": {
-          "daily": 7,
-          "weekly": 4,
-          "monthly": 3
-        }
-      }
-    },
     {
       "id": "gitea-backup",
       "repo": "hidrive-backup",