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.

gateway/internal/transcode/transcode.go

@@ -246,7 +246,7 @@ func gameStateHandler(backend *backendclient.Client) Handler {
 func enqueueHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsEnqueueRequest(req.Payload, 0)
-		m, err := backend.Enqueue(ctx, req.UserID, string(in.Variant()), in.MultipleWordsPerTurn())
+		m, err := backend.Enqueue(ctx, req.UserID, string(in.Variant()), in.MultipleWordsPerTurn(), in.VsAi())
 		if err != nil {
 			return nil, err
 		}