feat(lobby): enter the game immediately and wait for the opponent inside it

Quick auto-match no longer waits on a separate screen: Enqueue opens a real game seating the caller with an empty opponent seat (new game status 'open') and the player enters it at once. A second human searching the same variant+rule joins that open game; otherwise a background reaper seats a robot after a 90s + random 0-90s wait, pushing a new in-app opponent_joined event that fills the opponent card and re-enables resign and chat in place.

Matchmaking state is now the open games in the database (the in-memory pool, lobby.poll and lobby.cancel are gone), serialised by a per-bucket advisory lock. While a game is open the starter may move on their turn, but resign, chat and nudge are refused; the lobby and opponent card show "searching for opponent".

Schema edited in the baseline (no prod data): 'open' status, nullable game_players.account_id for the empty seat, and a games.open_deadline_at stamp; jet code regenerated.

backend/cmd/backend/main.go

@@ -170,12 +170,14 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	go robots.Run(ctx, cfg.Robot.DriveInterval)
 	logger.Info("robot driver started", zap.Duration("interval", cfg.Robot.DriveInterval))
 
-	matchmaker := lobby.NewMatchmaker(games, robots, cfg.Lobby.RobotWait, logger)
+	matchmaker := lobby.NewMatchmaker(games, robots, cfg.Lobby.RobotWait, cfg.Lobby.RobotWaitJitter, logger)
 	matchmaker.SetNotifier(hub)
 	go matchmaker.RunReaper(ctx, cfg.Lobby.ReaperInterval)
 	invitations := lobby.NewInvitationService(lobby.NewStore(db), games, accounts, socialSvc)
 	invitations.SetNotifier(hub)
-	logger.Info("lobby and social domains ready", zap.Duration("robot_wait", cfg.Lobby.RobotWait))
+	logger.Info("lobby and social domains ready",
+		zap.Duration("robot_wait", cfg.Lobby.RobotWait),
+		zap.Duration("robot_wait_jitter", cfg.Lobby.RobotWaitJitter))
 
 	// Rate-limit observability: ingest the gateway's rejection reports for the
 	// admin throttled view and the conservative high-rate auto-flag.