fix(migrations): 停用旧模型冲突别名

以可回滚的状态更新替代删除，满足生产迁移安全门禁。

apps/api/migrations/0079_image_model_migration_contract.sql

@@ -150,15 +150,19 @@ stable_models AS (
     ON stable.provider_key = 'gemini'
    AND stable.invocation_name = replacements.invocation_name
 ),
-removed_conflicting_aliases AS (
-  DELETE FROM model_compatibility_aliases compatibility_alias
-  USING preview_models preview, stable_models stable
+disabled_conflicting_aliases AS (
+  UPDATE model_compatibility_aliases compatibility_alias
+  SET active = false,
+      expires_at = COALESCE(compatibility_alias.expires_at, now()),
+      updated_at = now()
+  FROM preview_models preview, stable_models stable
   WHERE compatibility_alias.base_model_id = preview.id
     AND stable.preview_invocation_name = preview.invocation_name
     AND compatibility_alias.alias IN (
       stable.invocation_name,
       'gemini:' || preview.invocation_name
     )
+  RETURNING compatibility_alias.id
 )
 INSERT INTO model_compatibility_aliases (
   base_model_id,