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/api/routes/system.py

@@ -45,8 +45,7 @@ from wled_controller.core.capture.screen_capture import get_available_displays
 from wled_controller.utils import get_logger
 from wled_controller.storage.base_store import EntityNotFoundError
 
-# Re-export STORE_MAP and load_external_url so existing callers still work
-from wled_controller.api.routes.backup import STORE_MAP  # noqa: F401
+# Re-export load_external_url so existing callers still work
 from wled_controller.api.routes.system_settings import load_external_url  # noqa: F401
 
 logger = get_logger(__name__)