feat(rules): forbid repeating a word already on the board in Erudit

Russian "Эрудит" treats a word laid on the board as belonging to the
game: it cannot be laid again. Neither the solver, the backend nor the
offline JS port knew the rule, so a player (and the robot) could replay a
word freely. Official Scrabble places no such restriction, so both
Scrabble variants keep playing unrestricted.

The rule applies in two ways. A play whose main word is already on the
board is illegal, and is neither accepted nor generated. A play whose
perpendicular cross-word is already there stands — that word is
incidental to laying the main word — but scores nothing. The set of
played words is the game's own move journal, main words and cross-words
alike, compared decoded, so a word spelled with a blank is the same word.

It lives in the game layer, not the solver: only a game knows its
history, and the solver stays stateless and standard-rules. The backend
applies it at submit, at the move preview and over generated moves
(filtering and re-ranking them, so neither the robot nor the hint can
offer a play the engine would then refuse); the client port does the same
for the offline engine and for the on-device preview of an online game.

The rule is pinned per game (games.no_repeat_words, set from the variant
at creation) rather than keyed on the variant, because a game is replayed
from its journal on every open. Applied retroactively it would make an
already-played repeat illegal — closing that game as a draw — and would
rescore a play whose cross-word repeats an earlier word, shifting a live
game's totals. Games created before the rule keep playing without it. The
flag rides the wire as a trailing field because the client's preview must
score the way the server does, and offline games pin the same answer in
their own record.

backend/internal/game/service.go

@@ -284,6 +284,13 @@ func (svc *Service) versionResident(version string) bool {
 	return false
 }
 
+// noRepeatWordsFor reports whether a game of variant v is created with the no-repeat-words rule
+// (see engine/norepeat.go): a word already on the board cannot be laid again. It is a rule of
+// Russian "Эрудит" alone — both Scrabble variants let a word be played as often as a player can
+// manage. The answer is pinned into games.no_repeat_words at creation and read back from there
+// afterwards, never re-derived, so games created before the rule existed keep their own answer.
+func noRepeatWordsFor(v engine.Variant) bool { return v == engine.VariantErudit }
+
 // Create starts and persists a new game seating the given accounts in turn order
 // (seat 0 first), deals the racks, and warms the live-game cache. It validates
 // the player count (2–4), the move clock, the hint allowance and that every seat
@@ -359,6 +366,7 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 		Seed:                 seed,
 		DropoutTiles:         params.DropoutTiles,
 		MultipleWordsPerTurn: params.MultipleWordsPerTurn,
+		NoRepeatWords:        noRepeatWordsFor(params.Variant),
 	})
 	if err != nil {
 		if errors.Is(err, engine.ErrUnknownVariant) || errors.Is(err, engine.ErrUnknownVersion) {
@@ -382,6 +390,7 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 		hintsPerPlayer:       params.HintsPerPlayer,
 		dropoutTiles:         params.DropoutTiles.String(),
 		multipleWordsPerTurn: params.MultipleWordsPerTurn,
+		noRepeatWords:        noRepeatWordsFor(params.Variant),
 		vsAI:                 params.VsAI,
 		kind:                 params.Kind,
 	}
@@ -445,6 +454,7 @@ func (svc *Service) OpenOrJoin(ctx context.Context, accountID uuid.UUID, params
 		hintsPerPlayer:       params.HintsPerPlayer,
 		dropoutTiles:         params.DropoutTiles.String(),
 		multipleWordsPerTurn: params.MultipleWordsPerTurn,
+		noRepeatWords:        noRepeatWordsFor(params.Variant),
 		status:               StatusOpen,
 		openDeadline:         &deadline,
 		kind:                 gamelimits.KindRandom,
@@ -1584,6 +1594,7 @@ func (svc *Service) replay(ctx context.Context, pre Game) (*engine.Game, error)
 		Seed:                 seed,
 		DropoutTiles:         pre.DropoutTiles,
 		MultipleWordsPerTurn: pre.MultipleWordsPerTurn,
+		NoRepeatWords:        pre.NoRepeatWords,
 	})
 	if err != nil {
 		return nil, err