Add GNU GPLv3 license

Add the official FSF license text, a copyright/license header in
server.js, license metadata in package.json, and a License section in
the README. Also translate package.json's description field, missed
in the earlier English pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server.js

@@ -1,5 +1,12 @@
 'use strict';
 
+// Copyright (C) 2026 Dasemu
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version. See the LICENSE file for details.
+
 const crypto = require('crypto');
 const express = require('express');