The `matches` table has no `last_activity` or `ended_at` columns.
PostgREST rejects the entire PATCH when ANY column doesn't exist,
causing every move update and resignation to silently fail.

- Remove `last_activity` from handleGameMove (column doesn't exist)
- Change `ended_at` to `completed_at` in handleResign, handleDraw,
  handleComplete (correct column name per DB schema)

This was the root cause of zero sync in multiplayer chess.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>