feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

docs/FUNCTIONAL.md

@@ -31,13 +31,9 @@ ephemeral guest. The gateway validates the credential once and mints a thin
 session token; the backend resolves it to an internal `user_id`. A **Telegram Mini
 App** launch authenticates from the platform's signed `initData`, themes the UI to
 the Telegram colours, and — on first contact — seeds the new account's interface
-language from the Telegram client. The sign-in service also declares the **game
-languages** it offers (a set of en/ru, at least one), which gate the New Game variant
-choice in the lobby. Telegram runs a separate bot per language (an English bot and a
-Russian bot, the same player spanning both); the bot a player signed in through sets their
-offered languages, and their non-game notifications come from it. A **game's** notifications
-(your turn, game over, a nudge), though, always come from **that game's** bot — by the game's
-language, not whichever bot the player signed in through last. Guests are session-only with restricted features
+language from the Telegram client. Telegram runs a **single bot**: every player uses
+the same bot, and all of its chat and out-of-app notifications are written in the
+player's own **interface language** (en/ru). Guests are session-only with restricted features
 (auto-match only; no friends, stats or history); an abandoned guest that never
 joined a game and has been idle past the retention window is garbage-collected. While the app is open the client
 keeps a live stream and receives in-app updates in real time — the opponent's move,
@@ -85,12 +81,12 @@ in the other players' lists, and there is no undo. A finished **AI game (🤖) y
 resigning or by letting it lapse to the 7-day timeout — drops from your *finished* list
 automatically, with no swipe needed; a normally finished AI game stays until you remove it, and
 no other game type is auto-removed. The game types offered on **New Game** are
-limited to the languages the player's sign-in service supports (English → Scrabble;
-Russian → Scrabble + Erudite; a bilingual service shows all three, and the web client is
-unrestricted). Variants are shown by their **display name** — both Scrabble variants read
+limited to the player's chosen **variant preferences** (see *Profile & settings*) —
+Erudite-first, defaulting to Erudite only. Variants are shown by their **display name** — both Scrabble variants read
 "Scrabble"/"Скрэббл" and Erudit reads "Erudite"/"Эрудит" (by the interface language), and
-the same name titles the in-game screen. This gates only **starting** a new game — both auto-match and a friend
-invitation — so a player still sees and plays existing games of any language.
+the same name titles the in-game screen. This gates only **starting** a new game you initiate —
+auto-match, a vs-AI game and a friend invitation **you create** — so a player still sees and plays
+existing games of any variant, and being **invited** to a game lets you accept it in any variant.
 
 **Quick game** lets you choose your opponent — an **AI** (the default) or a **random player**.
 With **AI** you start at once against a 🤖 that joins and replies immediately: there is no waiting,
@@ -237,6 +233,15 @@ block toggles. The profile form is edited inline (no separate edit mode). Linkin
 an email or Telegram and merging accounts are covered under "Accounts, linking &
 merge".
 
+**Preferences (which variants you can be matched into).** A profile setting picks the game
+variants — Erudite, Russian Scrabble and English Scrabble, shown **Erudite-first** — you allow
+yourself to be matched into; a **new account starts with Erudite only**, and you must keep **at
+least one** selected. This list is exactly what **New Game** offers when you start a game
+(auto-match, an AI game, or a friend invitation you create) — a variant you have not enabled is
+not offered, and the server refuses it. It does not restrict games you are **invited** to: an
+invited friend may accept an invitation in **any** variant, and you can always open and play
+existing games of any variant.
+
 ### Feedback
 A registered player reaches the operators from Settings → Info: a **Feedback** screen with a
 message (up to 1024 characters) and an optional single attachment (one file, up to ~1 MB — images,