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/backup.go

@@ -514,7 +514,7 @@ func (m *Manager) RefreshCache(nextDBDump, nextBackup time.Time) {
 	// Discover app data (for per-app backup toggles)
 	if m.stackProvider != nil {
 		backupPrefs := m.settings.GetAppBackupMap()
-		status.AppDataInfo = DiscoverAppData(m.stackProvider, m.cfg.Paths.HDDPath, backupPrefs, status.DiscoveredDBs)
+		status.AppDataInfo = DiscoverAppData(m.stackProvider, backupPrefs, status.DiscoveredDBs)
 
 		// Include enabled app backup paths in the displayed BackupPaths
 		appPaths := m.resolveAppBackupPaths()