chore: add missing SPDX ID and workflow check (#13570)

* chore: add workflow which checks if SPDX ID exists

* chore: add missing SPDX ID in some files

* chore: change trigger condition

* chore: trigger on push

* lint

packages/backend/test/unit/misc/loader.ts

@@ -1,3 +1,8 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 import { DebounceLoader } from '@/misc/loader.js';
 
 class Mock {