Root cause: draw offers stored in game_state were lost when a
concurrent move write did read→merge→write (non-atomic). Now uses
PostgreSQL jsonb || operator via merge_game_state() RPC for atomic
merges.

Also adds:
- mp_log table for server-side multiplayer event logging
- Frontend sessionStorage logging (sessionStorage.mp_log) for
  debugging draw offers, moves, and poll events
- Logging on match_created, move, resign, draw_accepted, game_state_merge
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>