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/stores/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """Stores package for Golf game V2 persistence."""
 
 from .event_store import EventStore, ConcurrencyError

server/stores/event_store.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """
 PostgreSQL-backed event store for Golf game.
 

server/stores/pubsub.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """
 Redis pub/sub for cross-server game events.
 

server/stores/state_cache.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """
 Redis-backed live game state cache.
 

server/stores/user_store.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """
 PostgreSQL-backed user store for Golf game authentication.