- Add dequeue handler to backgammon-match.php (was missing)
- Add 90-second stale entry cleanup to ludo/domino/backgammon queues
- Add atomic opponent claiming (conditional update) to prevent race conditions
- Fix client queue.js to route dequeue to correct game endpoint
- Register unmountQueue for proper cleanup when exiting search screen
- Remove json_encode on jsonb fields in backgammon (prevent double-encoding)

Fixes: self-matching, ghost queue entries, and race condition where two
players simultaneously claim the same opponent.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>