chore: add AGPLv3 copyright header to all source files

- 52 JS files (backend + public/js): header with
  Copyright (C) 2026 Sea Network Technology Studio
  Author: CangLan <admin@sea-studio.top>
  + AGPLv3 notice
- idempotent (skips if header present), all syntax-checked

backend/middleware/security.js

@@ -1,3 +1,22 @@
+/*
+ * MC Report System
+ * Copyright (C) 2026 Sea Network Technology Studio
+ * Author: CangLan <admin@sea-studio.top>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 const xss = require('xss');
 const rateLimit = require('express-rate-limit');
 const fs = require('fs');