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>

README.md

@@ -91,3 +91,7 @@ docker run -d --name gitea-claude-webhook-proxy \
 
 Meant to sit behind an existing nginx doing `proxy_pass` to
 `http://127.0.0.1:3000/gitea-webhook`.
+
+## License
+
+[GNU GPLv3](LICENSE) or later.