fix: update down_revision in migration and correct import statement in app_asset model

api/models/app_asset.py

@@ -2,7 +2,7 @@ from datetime import datetime
 from uuid import uuid4
 
 import sqlalchemy as sa
-from sqlalchemy import DateTime, Integer, func
+from sqlalchemy import DateTime, Integer, String, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from core.app.entities.app_asset_entities import AppAssetFileTree