Fix migration (#4982)

Author: Weves

backend/alembic/versions/c9e2cd766c29_add_s3_file_store_table.py

@@ -241,6 +241,8 @@ def _migrate_files_to_external_storage() -> None:
     _set_tenant_contextvar(session)
     migrated_count = 0
 
+    external_store.initialize()
+
     for i, file_id in enumerate(files_to_migrate, 1):
         print(f"Migrating file {i}/{total_files}: {file_id}")