Add rollback after failed migration to allow subsequent migrations to run

When a migration fails, the transaction is left in a failed state. Without
rollback, all subsequent migrations fail with "current transaction is aborted".
This was preventing the team_id column from being added to projects.

backend/app/database.py

@@ -347,6 +347,7 @@ def _run_migrations():
                 conn.execute(text(migration))
                 conn.commit()
             except Exception as e:
+                conn.rollback()
                 logger.warning(f"Migration failed (may already be applied): {e}")