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.

ui/src/lib/variants.ts

@@ -11,14 +11,15 @@ export interface VariantOption {
   label: MessageKey;
 }
 
-// ALL_VARIANTS lists every variant in display order. The labels are display names keyed by
+// ALL_VARIANTS lists every variant in display order (Erudit first — the default
+// preference and the product's primary variant). The labels are display names keyed by
 // the game's alphabet, not the interface language: the English-alphabet game is always
 // "Scrabble" and the Russian-alphabet Scrabble always "Скрэббл" (both unlocalized, so the
 // two never collide whatever the UI language); Erudit is localized "Erudite"/"Эрудит".
 export const ALL_VARIANTS: VariantOption[] = [
-  { id: 'scrabble_en', label: 'new.english' },
-  { id: 'scrabble_ru', label: 'new.russian' },
   { id: 'erudit_ru', label: 'new.erudit' },
+  { id: 'scrabble_ru', label: 'new.russian' },
+  { id: 'scrabble_en', label: 'new.english' },
 ];
 
 // variantNameKey returns the i18n key for a variant's display name (used by the in-game
@@ -47,13 +48,13 @@ export const VARIANT_FLAG: Record<Variant, string> = {
 // ru -> Russian + Эрудит.
 export const VARIANT_LANGUAGE: Record<Variant, 'en' | 'ru'> = { scrabble_en: 'en', scrabble_ru: 'ru', erudit_ru: 'ru' };
 
-// availableVariants gates ALL_VARIANTS by the session's supported languages. An empty
-// or absent set is ungated (a web/legacy session without a declared set), returning
-// every variant.
-export function availableVariants(supportedLanguages: string[] | undefined): VariantOption[] {
-  const langs = supportedLanguages ?? [];
-  if (langs.length === 0) return ALL_VARIANTS;
-  return ALL_VARIANTS.filter((v) => langs.includes(VARIANT_LANGUAGE[v.id]));
+// availableVariants gates ALL_VARIANTS by the player's variant preferences (the set
+// they enabled in Settings). An empty or absent set is ungated (returns every variant)
+// — a safety fallback; a real profile always carries at least one preference.
+export function availableVariants(preferences: Variant[] | undefined): VariantOption[] {
+  const prefs = preferences ?? [];
+  if (prefs.length === 0) return ALL_VARIANTS;
+  return ALL_VARIANTS.filter((v) => prefs.includes(v.id));
 }
 
 // supportsMultipleWordsToggle reports whether the New Game "multiple words per turn" toggle