v0.9.0: Storage paths registry, per-app HDD_PATH resolution, storage management UI

- Fix backup toggles not appearing (read each app's own HDD_PATH from app.yaml)
- Storage paths registry in settings.json with auto-discovery from deployed apps
- Settings page "Adattárolók" section with disk usage, add/remove/default/schedulable
- Deploy page path field as dropdown of registered storage paths
- Health check storage monitoring (mount point, disk usage alerts)
- Mount-point validation utilities (Linux syscall + cross-platform stubs)
- Controller docker-compose mount changed to /mnt:/mnt:rw for multi-storage

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/configs/controller.yaml.example

@@ -31,7 +31,7 @@ paths:
   data_dir: "/opt/docker/felhom-controller/data"
   backup_dir: "/srv/backups"
   db_dump_dir: "/srv/backups/db-dumps"
-  hdd_path: ""                         # Optional: HDD mount path (e.g., /mnt/hdd)
+  hdd_path: ""                         # DEPRECATED: use Settings > Adattárolók instead. Fallback only for auto-discovery.
 
 # --- System ---
 system: