Fix alembic migration file missing import (#803)

Author: wu-clan

backend/alembic/script.py.mako

@@ -7,6 +7,7 @@ Create Date: ${create_date}
 """
 from alembic import op
 import sqlalchemy as sa
+import backend.common.model
 ${imports if imports else ""}
 
 # revision identifiers, used by Alembic.