Server pushes small change notes (messages / lobby / sheet) per lobby;
clients fetch data through the normal API so visibility rules stay in one
place. Polling stays as slow fallback. nginx WebSocket headers documented.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HJhXuLD2Z6hv3Wqeq9UL9K
