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.

docs/ARCHITECTURE.md

@@ -561,6 +561,24 @@ Key points:
   it on (standard). For auto-match the rule is part of the matchmaking key, so only players
   who chose the same rule are paired (the rule field rides every create/enqueue request, so
   matchmaking stays one uniform path).
+- **No repeated words (Erudit).** A word laid on the board cannot be laid again — the "Эрудит"
+  rule; official Scrabble has no such restriction, so both Scrabble variants never take it. It
+  applies in two ways: a play whose **main word** is already on the board is illegal, and a play
+  whose **cross-word** is already there stands (it is incidental to laying the main word) but that
+  cross-word scores nothing. The set of played words is the game's own **move journal** — the main
+  word and every cross-word of every play, compared decoded, so a word spelled with a blank is the
+  same word. The rule lives in the **game layer**, not the solver: `backend/internal/engine`
+  (`norepeat.go`) and its client port `ui/src/lib/dict/norepeat.ts` apply 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 solver stays stateless and
+  standard-rules; only a game knows its history.
+  It is **pinned per game** (`games.no_repeat_words`, set from the variant at creation and read
+  back thereafter, never re-derived) 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 (§9.1) — and would rescore a play whose cross-word repeats
+  an earlier word. Games created before the rule therefore keep playing without it. The flag rides
+  the wire (`GameView.no_repeat_words`) because the client's on-device move preview must score the
+  way the server does; offline games pin the same answer in their own record.
 - **End of game**: the bag is empty **and** a player empties their rack, **or**
   **6 consecutive scoreless turns** (passes/exchanges), **or** a resignation, or
   a missed turn. The **per-game turn timeout** is chosen at creation