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/FUNCTIONAL.md

@@ -196,7 +196,16 @@ the simplified **single-word rule** — only the word laid along the player's li
 real word (and it must still cross or extend letters already on the board along that line),
 and any incidental perpendicular words are ignored and not scored — while on is
 standard Scrabble. English games are always standard and show no such toggle. In auto-match
-the choice joins the pairing key, so a player only meets opponents who picked the same rule. Friend games (2–4) are
+the choice joins the pairing key, so a player only meets opponents who picked the same rule.
+
+**Erudite forbids repeating a word.** A word laid on the board belongs to the game: it cannot be laid
+again, and the AI never plays one either. A play whose main word is already on the board is rejected
+like any illegal move; a play that only forms an already-played word **across** its main word stands —
+that word is incidental to the move — but scores nothing. The rule is not a choice: every Erudite
+game takes it, and both Scrabble variants (where the official rules place no such restriction) never
+do, so the variant's one-line description on **New Game** reads "no repeated words". Games started
+before the rule existed keep playing without it, so a game in progress never changes rules under its
+players. Friend games (2–4) are
 formed by inviting players from the friend list (an invitation, like a friend code,
 is shareable as a Telegram deep link that opens it directly — on VK, which forwards no payload to the
 Mini App, the link only opens the app and the recipient enters the copied code by hand): the inviter chooses the