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.

pkg/fbs/scrabble.fbs

@@ -70,6 +70,9 @@ table GameView {
   // last_activity_unix is the lobby sort key: the current turn's start for an active
   // game, the finish time for a finished one.
   last_activity_unix:long;
+  // vs_ai marks an honest-AI game: the opponent is shown as 🤖 and chat/nudge/add-friend
+  // are disabled in the client. A robot-filled random game keeps vs_ai=false.
+  vs_ai:bool;
 }
 
 // MoveRecord is one decoded move (a committed play, or a hint preview).
@@ -289,11 +292,13 @@ table GameList {
 
 // --- lobby (authenticated) ---
 
-// EnqueueRequest joins the auto-match pool for a variant under a per-turn word rule.
+// EnqueueRequest starts a quick game for a variant under a per-turn word rule.
 // multiple_words_per_turn true is standard Scrabble; false is the single-word rule.
+// vs_ai true starts an honest-AI game against a robot instead of human auto-match.
 table EnqueueRequest {
   variant:string;
   multiple_words_per_turn:bool;
+  vs_ai:bool;
 }
 
 // MatchResult reports whether the caller has been paired into a game yet.