Document task DB access and unify background task DB handling

backend/app/tasks/history_sync.py

@@ -11,8 +11,8 @@ from typing import TYPE_CHECKING
 
 import structlog
 
-from app.db import open_db
 from app.services import history_service
+from app.tasks.db import task_db
 from app.utils.runtime_state import get_effective_settings
 
 if TYPE_CHECKING:
@@ -34,15 +34,13 @@ BACKFILL_WINDOW: int = 648000
 
 async def _run_sync_with_settings(settings: Settings) -> None:
     socket_path: str = settings.fail2ban_socket
-    db = await open_db(settings.database_path)
 
     try:
-        synced = await history_service.sync_from_fail2ban_db(db, socket_path)
+        async with task_db(settings) as db:
+            synced = await history_service.sync_from_fail2ban_db(db, socket_path)
         log.info("history_sync_complete", synced=synced)
     except Exception:
         log.exception("history_sync_failed")
-    finally:
-        await db.close()
 
 
 async def _run_sync(app: FastAPI) -> None: