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/web/server.go

@@ -94,6 +94,14 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		s.settingsNotificationsHandler(w, r)
 	case path == "/settings/notifications/test" && r.Method == http.MethodPost:
 		s.settingsNotificationsTestHandler(w, r)
+	case path == "/settings/storage/add" && r.Method == http.MethodPost:
+		s.settingsStorageAddHandler(w, r)
+	case path == "/settings/storage/remove" && r.Method == http.MethodPost:
+		s.settingsStorageRemoveHandler(w, r)
+	case path == "/settings/storage/default" && r.Method == http.MethodPost:
+		s.settingsStorageDefaultHandler(w, r)
+	case path == "/settings/storage/schedulable" && r.Method == http.MethodPost:
+		s.settingsStorageSchedulableHandler(w, r)
 	case path == "/settings/app-backup" && r.Method == http.MethodPost:
 		s.settingsAppBackupHandler(w, r)
 	case path == "/backup/restore" && r.Method == http.MethodPost:
@@ -122,6 +130,14 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// primaryHDDPath returns the first registered storage path, or the legacy config value.
+func (s *Server) primaryHDDPath() string {
+	if paths := s.settings.GetStoragePaths(); len(paths) > 0 {
+		return paths[0].Path
+	}
+	return s.cfg.Paths.HDDPath
+}
+
 func (s *Server) render(w http.ResponseWriter, name string, data interface{}) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	if err := s.tmpl.ExecuteTemplate(w, name, data); err != nil {