chore(backend): missing SPDX

packages/backend/src/models/MahjongGame.ts

@@ -1,3 +1,8 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 import { PrimaryColumn, Entity, Index, JoinColumn, Column, ManyToOne } from 'typeorm';
 import { id } from './util/id.js';
 import { MiUser } from './User.js';