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.

deploy/.env.example

@@ -29,26 +29,18 @@ GM_BASICAUTH_HASH=                     # required; `caddy hash-password` bcrypt
 
 # --- UI build args (baked into the gateway image) ---------------------------
 VITE_TELEGRAM_BOT_ID=
-VITE_TELEGRAM_LINK=                                 # fallback friend-invite Mini App link (per-bot below)
-VITE_TELEGRAM_LINK_EN=                              # friend-invite Mini App link, English bot (full URL, e.g. https://t.me/<bot>/<app>)
-VITE_TELEGRAM_LINK_RU=                              # friend-invite Mini App link, Russian bot (full URL)
-VITE_TELEGRAM_GAME_CHANNEL_NAME_EN=                 # landing "Play in Telegram" link, English bot
-VITE_TELEGRAM_GAME_CHANNEL_NAME_RU=                 # landing "Play in Telegram" link, Russian bot
+VITE_TELEGRAM_LINK=                                 # friend-invite Mini App link (full URL, e.g. https://t.me/<bot>/<app>)
+VITE_TELEGRAM_GAME_CHANNEL_NAME=                    # landing "Play in Telegram" link, the bot's game channel
 VITE_GATEWAY_URL=
 
-# --- Gateway ----------------------------------------------------------------
-GATEWAY_DEFAULT_SUPPORTED_LANGUAGES=en,ru
-
 # --- Grafana ----------------------------------------------------------------
 GRAFANA_ROOT_URL=/_gm/grafana/         # set the full https URL behind a real domain
 GRAFANA_ADMIN_PASSWORD=admin
 
 # --- Telegram connector -----------------------------------------------------
 AWG_CONF=                              # required; AmneziaWG sidecar config
-TELEGRAM_BOT_TOKEN_EN=                 # at least one of EN/RU required
-TELEGRAM_BOT_TOKEN_RU=
-TELEGRAM_GAME_CHANNEL_ID_EN=
-TELEGRAM_GAME_CHANNEL_ID_RU=
+TELEGRAM_BOT_TOKEN=                    # required
+TELEGRAM_GAME_CHANNEL_ID=
 TELEGRAM_MINIAPP_URL=                  # required
 TELEGRAM_TEST_ENV=false
 TELEGRAM_API_BASE_URL=