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/docker-compose.yml

@@ -21,8 +21,8 @@ services:
       - /opt/docker/stacks:/opt/docker/stacks
       # Backup directories (restic repo + db dumps)
       - /srv/backups:/srv/backups
-      # HDD mount (if available, for monitoring disk usage)
-      - ${HDD_PATH:-/mnt/hdd_placeholder}:${HDD_PATH:-/mnt/hdd_placeholder}:ro
+      # All external storage — /mnt/* for multi-storage + restore
+      - /mnt:/mnt:rw
       # Host /sys — for CPU temperature reading (read-only)
       - /sys:/host/sys:ro
       # Host OS info — for monitoring page system info