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/app.ts

@@ -191,10 +191,9 @@ import {
 import {
     openSettingsModal, closeSettingsModal, switchSettingsTab,
     downloadBackup, handleRestoreFileSelected,
-    saveAutoBackupSettings, restoreSavedBackup, downloadSavedBackup, deleteSavedBackup,
+    saveAutoBackupSettings, triggerBackupNow, restoreSavedBackup, downloadSavedBackup, deleteSavedBackup,
     restartServer, saveMqttSettings,
     loadApiKeysList,
-    downloadPartialExport, handlePartialImportFileSelected,
     connectLogViewer, disconnectLogViewer, clearLogViewer, applyLogFilter,
     openLogOverlay, closeLogOverlay,
     loadLogLevel, setLogLevel,
@@ -536,21 +535,20 @@ Object.assign(window, {
     openCommandPalette,
     closeCommandPalette,
 
-    // settings (tabs / backup / restore / auto-backup / MQTT / partial export-import / api keys / log level)
+    // settings (tabs / backup / restore / auto-backup / MQTT / api keys / log level)
     openSettingsModal,
     closeSettingsModal,
     switchSettingsTab,
     downloadBackup,
     handleRestoreFileSelected,
     saveAutoBackupSettings,
+    triggerBackupNow,
     restoreSavedBackup,
     downloadSavedBackup,
     deleteSavedBackup,
     restartServer,
     saveMqttSettings,
     loadApiKeysList,
-    downloadPartialExport,
-    handlePartialImportFileSelected,
     connectLogViewer,
     disconnectLogViewer,
     clearLogViewer,