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/scrabblefb/EnqueueRequest.go

@@ -61,8 +61,20 @@ func (rcv *EnqueueRequest) MutateMultipleWordsPerTurn(n bool) bool {
 	return rcv._tab.MutateBoolSlot(6, n)
 }
 
+func (rcv *EnqueueRequest) VsAi() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *EnqueueRequest) MutateVsAi(n bool) bool {
+	return rcv._tab.MutateBoolSlot(8, n)
+}
+
 func EnqueueRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(2)
+	builder.StartObject(3)
 }
 func EnqueueRequestAddVariant(builder *flatbuffers.Builder, variant flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(variant), 0)
@@ -70,6 +82,9 @@ func EnqueueRequestAddVariant(builder *flatbuffers.Builder, variant flatbuffers.
 func EnqueueRequestAddMultipleWordsPerTurn(builder *flatbuffers.Builder, multipleWordsPerTurn bool) {
 	builder.PrependBoolSlot(1, multipleWordsPerTurn, false)
 }
+func EnqueueRequestAddVsAi(builder *flatbuffers.Builder, vsAi bool) {
+	builder.PrependBoolSlot(2, vsAi, false)
+}
 func EnqueueRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }