Release readiness probe database transactions

src/user_engine/adapters/postgres.py

@@ -591,7 +591,12 @@ class PostgresUserEngineStore:
                     """,
                     (LATEST_SCHEMA_VERSION,),
                 )
-                return cursor.fetchone() is not None
+                present = cursor.fetchone() is not None
+            # psycopg starts a transaction even for this readiness SELECT.
+            # End it immediately so probes cannot retain relation locks that
+            # block the next replica's idempotent schema migration.
+            self.connection.rollback()
+            return present
         except Exception:
             self.connection.rollback()
             return False