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.

PLAN.md

@@ -281,6 +281,12 @@ back to `preferred_language`). Non-Telegram logins (web/email/guest) carry the g
 (`GATEWAY_DEFAULT_SUPPORTED_LANGUAGES`, all variants). Admin broadcasts (`SendToUser`/`SendToGameChannel`)
 pick the bot by an **operator-chosen** language in the console — unrelated to `ValidateInitData`.
 
+> **Superseded (2026-06-20, pre-release phase SB):** the two bots collapsed into **one** and the
+> language-gated variant choice moved to a per-user **`variant_preferences`** profile set (default Erudit
+> only). `accounts.service_language`, `supported_languages`, the `*_EN`/`*_RU` env vars,
+> `GATEWAY_DEFAULT_SUPPORTED_LANGUAGES` and game-language push routing are gone; the single bot renders in
+> the recipient's `preferred_language`. Current state: `docs/ARCHITECTURE.md` §3/§10, `PRERELEASE.md` row SB.
+
 ### Stage 16 — Deploy infra & test contour *(done)*
 Scope: the deploy machinery + the **test contour** (the bulk of the original Stage 14). Backend +
 gateway **Dockerfiles** (multi-stage distroless, mirroring the Stage 9 connector image); the gateway