feat: honest AI opponent in quick game

New Game's quick game gains an explicit opponent selector — 🤖 AI (default)
or 👤 Random player. AI starts a game seated with a pooled robot that joins
and moves at once: no per-move timeout (a 7-day inactivity loss reusing the
turn-timeout sweeper), chat/nudge disabled, no statistics, the opponent shown
as 🤖 everywhere. The random path (disguised robot) is unchanged.

Driven by one game flag (games.vs_ai), set only on AI-started games so the
disguised path is never revealed; Matchmaker.StartVsAI seats the robot
directly (no open pool); the robot replies event-driven via the game service's
after-commit/after-create hook. Wire: vs_ai on EnqueueRequest + GameView.

backend/cmd/backend/main.go

@@ -178,6 +178,9 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	if err := robots.EnsurePool(ctx); err != nil {
 		return fmt.Errorf("provision robot pool: %w", err)
 	}
+	// Honest-AI fast path: a move in a vs_ai game triggers the robot's reply at once
+	// (the periodic driver below is the fallback). Set after the pool is provisioned.
+	games.SetAITrigger(robots.TriggerMove)
 	go robots.Run(ctx, cfg.Robot.DriveInterval)
 	logger.Info("robot driver started", zap.Duration("interval", cfg.Robot.DriveInterval))