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/game/store.go

@@ -38,6 +38,8 @@ type gameInsert struct {
 	hintsAllowed    bool
 	hintsPerPlayer  int
 	dropoutTiles    string
+	// multipleWordsPerTurn false selects the single-word rule for the game.
+	multipleWordsPerTurn bool
 }
 
 // statDelta is one account's contribution to its statistics on a game finish.
@@ -92,8 +94,8 @@ func (s *Store) CreateGame(ctx context.Context, ins gameInsert, seats []uuid.UUI
 		gi := table.Games.INSERT(
 			table.Games.GameID, table.Games.Variant, table.Games.DictVersion, table.Games.Seed,
 			table.Games.Players, table.Games.TurnTimeoutSecs, table.Games.HintsAllowed, table.Games.HintsPerPlayer,
-			table.Games.DropoutTiles,
-		).VALUES(ins.id, ins.variant, ins.dictVersion, ins.seed, ins.players, ins.turnTimeoutSecs, ins.hintsAllowed, ins.hintsPerPlayer, ins.dropoutTiles)
+			table.Games.DropoutTiles, table.Games.MultipleWordsPerTurn,
+		).VALUES(ins.id, ins.variant, ins.dictVersion, ins.seed, ins.players, ins.turnTimeoutSecs, ins.hintsAllowed, ins.hintsPerPlayer, ins.dropoutTiles, ins.multipleWordsPerTurn)
 		if _, err := gi.ExecContext(ctx, tx); err != nil {
 			return fmt.Errorf("insert game: %w", err)
 		}
@@ -761,6 +763,7 @@ func projectGame(g model.Games, seats []model.GamePlayers) (Game, error) {
 		CreatedAt:      g.CreatedAt,
 		UpdatedAt:      g.UpdatedAt,
 	}
+	out.MultipleWordsPerTurn = g.MultipleWordsPerTurn
 	if g.EndReason != nil {
 		out.EndReason = *g.EndReason
 	}