fix: skip enum upgrade before initial schema creation

backend/migrations/versions/20260803_reliability_monitoring.py

@@ -118,6 +118,12 @@ def _upgrade_postgresql_alert_type(connection: Connection) -> None:
     """Permit the degraded alert value for databases using PostgreSQL enums."""
     if connection.dialect.name == "postgresql":
         enum_name = AlertEvent.__table__.c.alert_type.type.name
+        enum_exists = connection.execute(
+            text("SELECT 1 FROM pg_type WHERE typname = :enum_name"),
+            {"enum_name": enum_name},
+        ).scalar_one_or_none()
+        if enum_exists is None:
+            return
         connection.execute(
             text(
                 f"ALTER TYPE {_quote(connection, enum_name)} "