fix: import UTC in import_generation_from_zip function scope

The function-local import was missing UTC, causing NameError at runtime
when datetime.now(UTC) was called.

backend/services/export_import.py

@@ -347,7 +347,7 @@ async def import_generation_from_zip(file_bytes: bytes, db: Session) -> dict:
     from pathlib import Path
     import tempfile
     import shutil
-    from datetime import datetime
+    from datetime import UTC, datetime
     from .. import config
     
     zip_buffer = io.BytesIO(file_bytes)