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_invitations.go

@@ -27,17 +27,18 @@ type invitationInviteeDTO struct {
 
 // invitationDTO is a friend-game invitation with its settings and invitees.
 type invitationDTO struct {
-	ID              string                 `json:"id"`
-	Inviter         accountRefDTO          `json:"inviter"`
-	Invitees        []invitationInviteeDTO `json:"invitees"`
-	Variant         string                 `json:"variant"`
-	TurnTimeoutSecs int                    `json:"turn_timeout_secs"`
-	HintsAllowed    bool                   `json:"hints_allowed"`
-	HintsPerPlayer  int                    `json:"hints_per_player"`
-	DropoutTiles    string                 `json:"dropout_tiles"`
-	Status          string                 `json:"status"`
-	GameID          string                 `json:"game_id,omitempty"`
-	ExpiresAtUnix   int64                  `json:"expires_at_unix"`
+	ID                   string                 `json:"id"`
+	Inviter              accountRefDTO          `json:"inviter"`
+	Invitees             []invitationInviteeDTO `json:"invitees"`
+	Variant              string                 `json:"variant"`
+	TurnTimeoutSecs      int                    `json:"turn_timeout_secs"`
+	HintsAllowed         bool                   `json:"hints_allowed"`
+	HintsPerPlayer       int                    `json:"hints_per_player"`
+	DropoutTiles         string                 `json:"dropout_tiles"`
+	MultipleWordsPerTurn bool                   `json:"multiple_words_per_turn"`
+	Status               string                 `json:"status"`
+	GameID               string                 `json:"game_id,omitempty"`
+	ExpiresAtUnix        int64                  `json:"expires_at_unix"`
 }
 
 // invitationListDTO is the caller's open invitations.
@@ -47,27 +48,29 @@ type invitationListDTO struct {
 
 // createInvitationRequest proposes a friend game to the named invitees.
 type createInvitationRequest struct {
-	InviteeIDs      []string `json:"invitee_ids"`
-	Variant         string   `json:"variant"`
-	TurnTimeoutSecs int      `json:"turn_timeout_secs"`
-	HintsAllowed    bool     `json:"hints_allowed"`
-	HintsPerPlayer  int      `json:"hints_per_player"`
-	DropoutTiles    string   `json:"dropout_tiles"`
+	InviteeIDs           []string `json:"invitee_ids"`
+	Variant              string   `json:"variant"`
+	TurnTimeoutSecs      int      `json:"turn_timeout_secs"`
+	HintsAllowed         bool     `json:"hints_allowed"`
+	HintsPerPlayer       int      `json:"hints_per_player"`
+	DropoutTiles         string   `json:"dropout_tiles"`
+	MultipleWordsPerTurn bool     `json:"multiple_words_per_turn"`
 }
 
 // invitationDTOFrom projects a lobby invitation, resolving names through memo.
 func (s *Server) invitationDTOFrom(ctx context.Context, inv lobby.Invitation, memo map[string]string) invitationDTO {
 	dto := invitationDTO{
-		ID:              inv.ID.String(),
-		Inviter:         s.namedRef(ctx, inv.InviterID, memo),
-		Invitees:        make([]invitationInviteeDTO, 0, len(inv.Invitees)),
-		Variant:         inv.Settings.Variant.String(),
-		TurnTimeoutSecs: int(inv.Settings.TurnTimeout.Seconds()),
-		HintsAllowed:    inv.Settings.HintsAllowed,
-		HintsPerPlayer:  inv.Settings.HintsPerPlayer,
-		DropoutTiles:    inv.Settings.DropoutTiles.String(),
-		Status:          inv.Status,
-		ExpiresAtUnix:   inv.ExpiresAt.Unix(),
+		ID:                   inv.ID.String(),
+		Inviter:              s.namedRef(ctx, inv.InviterID, memo),
+		Invitees:             make([]invitationInviteeDTO, 0, len(inv.Invitees)),
+		Variant:              inv.Settings.Variant.String(),
+		TurnTimeoutSecs:      int(inv.Settings.TurnTimeout.Seconds()),
+		HintsAllowed:         inv.Settings.HintsAllowed,
+		HintsPerPlayer:       inv.Settings.HintsPerPlayer,
+		DropoutTiles:         inv.Settings.DropoutTiles.String(),
+		MultipleWordsPerTurn: inv.Settings.MultipleWordsPerTurn,
+		Status:               inv.Status,
+		ExpiresAtUnix:        inv.ExpiresAt.Unix(),
 	}
 	if inv.GameID != nil {
 		dto.GameID = inv.GameID.String()
@@ -102,9 +105,10 @@ func (s *Server) handleCreateInvitation(c *gin.Context) {
 		return
 	}
 	settings := lobby.InvitationSettings{
-		Variant:        variant,
-		HintsAllowed:   req.HintsAllowed,
-		HintsPerPlayer: req.HintsPerPlayer,
+		Variant:              variant,
+		HintsAllowed:         req.HintsAllowed,
+		HintsPerPlayer:       req.HintsPerPlayer,
+		MultipleWordsPerTurn: req.MultipleWordsPerTurn,
 	}
 	if req.TurnTimeoutSecs > 0 {
 		settings.TurnTimeout = time.Duration(req.TurnTimeoutSecs) * time.Second