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/storage/automation_store.py

@@ -1,27 +1,28 @@
-"""Automation storage using JSON files."""
+"""Automation storage using SQLite."""
 
 import uuid
 from datetime import datetime, timezone
 from typing import List, Optional
 
 from wled_controller.storage.automation import Automation, Condition
-from wled_controller.storage.base_store import BaseJsonStore
+from wled_controller.storage.base_sqlite_store import BaseSqliteStore
+from wled_controller.storage.database import Database
 from wled_controller.utils import get_logger
 
 logger = get_logger(__name__)
 
 
-class AutomationStore(BaseJsonStore[Automation]):
-    _json_key = "automations"
+class AutomationStore(BaseSqliteStore[Automation]):
+    _table_name = "automations"
     _entity_name = "Automation"
 
-    def __init__(self, file_path: str):
-        super().__init__(file_path, Automation.from_dict)
+    def __init__(self, db: Database):
+        super().__init__(db, Automation.from_dict)
 
     # Backward-compatible aliases
-    get_all_automations = BaseJsonStore.get_all
-    get_automation = BaseJsonStore.get
-    delete_automation = BaseJsonStore.delete
+    get_all_automations = BaseSqliteStore.get_all
+    get_automation = BaseSqliteStore.get
+    delete_automation = BaseSqliteStore.delete
 
     def create_automation(
         self,
@@ -56,7 +57,7 @@ class AutomationStore(BaseJsonStore[Automation]):
         )
 
         self._items[automation_id] = automation
-        self._save()
+        self._save_item(automation_id, automation)
         logger.info(f"Created automation: {name} ({automation_id})")
         return automation
 
@@ -93,6 +94,6 @@ class AutomationStore(BaseJsonStore[Automation]):
             automation.tags = tags
 
         automation.updated_at = datetime.now(timezone.utc)
-        self._save()
+        self._save_item(automation_id, automation)
         logger.info(f"Updated automation: {automation_id}")
         return automation