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/locales/en.json

@@ -1591,6 +1591,9 @@
     "settings.auto_backup.save": "Save Settings",
     "settings.auto_backup.saved": "Auto-backup settings saved",
     "settings.auto_backup.save_error": "Failed to save auto-backup settings",
+    "settings.auto_backup.backup_now": "Backup Now",
+    "settings.auto_backup.backup_created": "Backup created",
+    "settings.auto_backup.backup_error": "Backup failed",
     "settings.auto_backup.last_backup": "Last backup",
     "settings.auto_backup.never": "Never",
     "settings.saved_backups.label": "Saved Backups",