Fix purge_seed_data type mismatch for access_permissions.user_id (#107)

AccessPermission.user_id is VARCHAR (stores username), not UUID.
Changed to compare with user.username instead of user.id.

backend/app/purge_seed_data.py

@@ -194,7 +194,8 @@ def purge_seed_data(db: Session) -> dict:
             synchronize_session=False
         )
         # Delete any access permissions for this user
-        db.query(AccessPermission).filter(AccessPermission.user_id == user.id).delete(
+        # Note: AccessPermission.user_id is VARCHAR (username), not UUID
+        db.query(AccessPermission).filter(AccessPermission.user_id == user.username).delete(
             synchronize_session=False
         )
         db.delete(user)