Phase 1.6: add pollInFlight guard to match-session.js so overlapping polls
are skipped when server is slow.

Phase 2.5: room code generation retries up to 5 times on collision.

Phase 2.6: join_room uses service key, checks if room is full, prevents
double-join, and auto-starts when player count is met.

Phase 3.1: dealAndSyncToServer retries on failure with toast notification.

Phase 3.3: drawFromBoneyard syncs once after all tiles drawn instead of
firing N parallel requests.

Phase 3.6: check-invites uses PostgREST cs filter on JSONB players array
instead of scanning all waiting matches.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>