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/zh.json

@@ -1518,6 +1518,9 @@
     "settings.auto_backup.save": "保存设置",
     "settings.auto_backup.saved": "自动备份设置已保存",
     "settings.auto_backup.save_error": "保存自动备份设置失败",
+    "settings.auto_backup.backup_now": "立即备份",
+    "settings.auto_backup.backup_created": "备份已创建",
+    "settings.auto_backup.backup_error": "备份失败",
     "settings.auto_backup.last_backup": "上次备份",
     "settings.auto_backup.never": "从未",
     "settings.saved_backups.label": "已保存的备份",