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.

gateway/internal/backendclient/api.go

@@ -245,11 +245,12 @@ func (c *Client) GameState(ctx context.Context, userID, gameID string, includeAl
 	return out, err
 }
 
-// Enqueue joins the auto-match pool for a variant.
-func (c *Client) Enqueue(ctx context.Context, userID, variant string) (MatchResp, error) {
+// Enqueue joins the auto-match pool for a variant under a per-turn word rule
+// (multipleWords true is standard Scrabble, false the single-word rule).
+func (c *Client) Enqueue(ctx context.Context, userID, variant string, multipleWords bool) (MatchResp, error) {
 	var out MatchResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/user/lobby/enqueue", userID, "",
-		map[string]string{"variant": variant}, &out)
+		map[string]any{"variant": variant, "multiple_words_per_turn": multipleWords}, &out)
 	return out, err
 }