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/server/handlers_user.go

@@ -133,9 +133,10 @@ func (s *Server) handleGameState(c *gin.Context) {
 	c.JSON(http.StatusOK, dto)
 }
 
-// enqueueRequest joins the per-variant auto-match pool.
+// enqueueRequest joins the per-variant auto-match pool under a per-turn word rule.
 type enqueueRequest struct {
-	Variant string `json:"variant"`
+	Variant              string `json:"variant"`
+	MultipleWordsPerTurn bool   `json:"multiple_words_per_turn"`
 }
 
 // handleEnqueue joins the auto-match pool for a variant.
@@ -155,7 +156,7 @@ func (s *Server) handleEnqueue(c *gin.Context) {
 		abortBadRequest(c, "unknown variant")
 		return
 	}
-	res, err := s.matchmaker.Enqueue(c.Request.Context(), uid, variant)
+	res, err := s.matchmaker.Enqueue(c.Request.Context(), uid, variant, req.MultipleWordsPerTurn)
 	if err != nil {
 		s.abortErr(c, err)
 		return