feat: migrate storage from JSON files to SQLite

Replace 22 individual JSON store files with a single SQLite database
(data/ledgrab.db). All entity stores now use BaseSqliteStore backed by
SQLite with WAL mode, write-through caching, and thread-safe access.

- Add Database class with SQLite backup/restore API
- Add BaseSqliteStore as drop-in replacement for BaseJsonStore
- Convert all 16 entity stores to SQLite
- Move global settings (MQTT, external URL, auto-backup) to SQLite
  settings table
- Replace JSON backup/restore with SQLite snapshot backups (.db files)
- Remove partial export/import feature (backend + frontend)
- Update demo seed to write directly to SQLite
- Add "Backup Now" button to settings UI
- Remove StorageConfig file path fields (single database_file remains)

server/src/wled_controller/static/js/global.d.ts

@@ -358,14 +358,13 @@ interface Window {
     downloadBackup: (...args: any[]) => any;
     handleRestoreFileSelected: (...args: any[]) => any;
     saveAutoBackupSettings: (...args: any[]) => any;
+    triggerBackupNow: (...args: any[]) => any;
     restoreSavedBackup: (...args: any[]) => any;
     downloadSavedBackup: (...args: any[]) => any;
     deleteSavedBackup: (...args: any[]) => any;
     restartServer: (...args: any[]) => any;
     saveMqttSettings: (...args: any[]) => any;
     loadApiKeysList: (...args: any[]) => any;
-    downloadPartialExport: (...args: any[]) => any;
-    handlePartialImportFileSelected: (...args: any[]) => any;
     connectLogViewer: (...args: any[]) => any;
     disconnectLogViewer: (...args: any[]) => any;
     clearLogViewer: (...args: any[]) => any;