The CPU turn chain was awaited inline inside game_lock, blocking the
WebSocket message loop. The end_game message couldn't be processed
until all CPU turns finished. Now check_and_run_cpu_turn launches a
background task and returns immediately, keeping the message loop
responsive. The end_game and leave handlers cancel the task on demand.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>