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/postgres/migrations/00001_baseline.sql

@@ -97,6 +97,7 @@ CREATE TABLE games (
     updated_at        timestamptz NOT NULL DEFAULT now(),
     finished_at       timestamptz,
     dropout_tiles     text        NOT NULL DEFAULT 'remove',
+    multiple_words_per_turn boolean NOT NULL DEFAULT true,
     CONSTRAINT games_variant_chk CHECK (variant IN ('scrabble_en', 'scrabble_ru', 'erudit_ru')),
     CONSTRAINT games_status_chk CHECK (status IN ('active', 'finished')),
     CONSTRAINT games_players_chk CHECK (players BETWEEN 2 AND 4),
@@ -260,6 +261,7 @@ CREATE TABLE game_invitations (
     hints_allowed     boolean     NOT NULL DEFAULT true,
     hints_per_player  smallint    NOT NULL DEFAULT 1,
     dropout_tiles     text        NOT NULL DEFAULT 'remove',
+    multiple_words_per_turn boolean NOT NULL DEFAULT true,
     status            text        NOT NULL DEFAULT 'pending',
     game_id           uuid        REFERENCES games (game_id) ON DELETE SET NULL,
     expires_at        timestamptz NOT NULL,