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>

pyproject.toml

@@ -4,7 +4,7 @@ version = "3.1.6"
 description = "6-Card Golf card game with AI opponents"
 readme = "README.md"
 requires-python = ">=3.11"
-license = {text = "MIT"}
+license = {text = "GPL-3.0-or-later"}
 authors = [
     {name = "alee"}
 ]
@@ -13,7 +13,7 @@ classifiers = [
     "Development Status :: 3 - Alpha",
     "Framework :: FastAPI",
     "Intended Audience :: End Users/Desktop",
-    "License :: OSI Approved :: MIT License",
+    "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",