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/internal/backup/appdata.go

@@ -53,7 +53,7 @@ type AppDockerVolume struct {
 }
 
 // DiscoverAppData discovers backup-relevant data for all deployed apps.
-func DiscoverAppData(provider StackDataProvider, hddPath string, backupPrefs map[string]bool, discoveredDBs []DiscoveredDB) []AppBackupInfo {
+func DiscoverAppData(provider StackDataProvider, backupPrefs map[string]bool, discoveredDBs []DiscoveredDB) []AppBackupInfo {
 	if provider == nil {
 		return nil
 	}