feat: "multiple words per turn" rule for Russian games

Add a per-game rule chosen on New Game for Russian variants (default off = the
single-word rule; on = standard Scrabble). Off, only the main word along the play
direction is validated and scored; perpendicular cross-words are ignored,
including in robot move generation. The rule rides every create and enqueue
request and joins the matchmaking key, so games and auto-match stay one uniform
path; "Russian-only" is a UI affordance (English always sends standard and shows
no toggle).

- Engine: consume scrabble-solver v1.1.0's PlayOptions{IgnoreCrossWords}, threaded
  through engine.Options.MultipleWordsPerTurn -> playOpts() into validate, score
  and generate.
- Backend: thread the flag through game CreateParams/Game + store (games column),
  lobby InvitationSettings + invitation row, and the matchmaker queue key (variant
  + rule); persisted, so a rebuilt-from-journal game keeps it. Baseline migration
  gains multiple_words_per_turn (DB not versioned); jet regenerated.
- Edge: multiple_words_per_turn added to the EnqueueRequest / CreateInvitationRequest
  FlatBuffers tables (Go + TS regenerated) and threaded through the gateway.
- UI: a "Multiple words per turn" toggle on New Game, shown for Russian variants
  only (auto-match and friend invite), default off; English silently sends standard.
- Tests: backend engine/matchmaker; UI unit (gating) + Playwright e2e (solver
  corner-case + GCG fixtures ship in v1.1.0). Docs + PRERELEASE tracker updated.

backend/internal/inttest/lobby_test.go

@@ -35,14 +35,14 @@ func TestMatchmakingPairsAndStartsGame(t *testing.T) {
 	mm := newMatchmaker(t, newRobotService(t, newGameService()), 10*time.Second)
 	a, b := provisionAccount(t), provisionAccount(t)
 
-	r1, err := mm.Enqueue(ctx, a, engine.VariantEnglish)
+	r1, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue a: %v", err)
 	}
 	if r1.Matched {
 		t.Fatal("first enqueue must wait")
 	}
-	r2, err := mm.Enqueue(ctx, b, engine.VariantEnglish)
+	r2, err := mm.Enqueue(ctx, b, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue b: %v", err)
 	}

backend/internal/inttest/robot_test.go

@@ -150,7 +150,7 @@ func TestMatchmakerSubstitutesRobotEndToEnd(t *testing.T) {
 	human := provisionAccount(t)
 
 	before := time.Now()
-	r, err := mm.Enqueue(ctx, human, engine.VariantEnglish)
+	r, err := mm.Enqueue(ctx, human, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}