Fix missing import for Base and engine in migration script

scripts/migrate_csv_to_db.py

@@ -26,6 +26,7 @@ import pandas as pd
 import requests
 
 from backend.config import settings
+from backend.database import Base, engine, get_db_session
 from backend.models import (
     RESULT_FIELD_MAPPING,
     SCHOOL_FIELD_MAPPING,