Relicense project under GPL-3.0-or-later

Replaces the previous MIT license with GPL-3.0-or-later. Adds the full
GPL-3.0 license text at LICENSE, updates pyproject.toml metadata and
classifier, updates the README, and adds SPDX-License-Identifier headers
to all first-party server Python and client JavaScript sources.
Third-party anime.min.js is left untouched.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/scripts/create_admin.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-3.0-or-later
 """
 Create an admin user for the Golf game.