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/internal/robot/robot.go

@@ -39,6 +39,7 @@ var ErrNoRobotAvailable = errors.New("robot: no robot available in the pool")
 // player. game.Service satisfies it.
 type GameDriver interface {
 	RobotTurns(ctx context.Context, robotIDs []uuid.UUID) ([]game.RobotTurn, error)
+	RobotTurn(ctx context.Context, gameID uuid.UUID, robotIDs []uuid.UUID) (game.RobotTurn, bool, error)
 	Participants(ctx context.Context, gameID uuid.UUID) ([]uuid.UUID, int, string, error)
 	Candidates(ctx context.Context, gameID, accountID uuid.UUID) ([]engine.MoveRecord, error)
 	GameState(ctx context.Context, gameID, accountID uuid.UUID) (game.StateView, error)