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.

backend/internal/inttest/seat_display_name_test.go

@@ -37,7 +37,7 @@ func TestSeatNameFrozenAfterRename(t *testing.T) {
 
 	starter := provisionAccount(t)
 	if _, err := accounts.UpdateProfile(ctx, starter, account.ProfileUpdate{
-		DisplayName: "Original Name", PreferredLanguage: "en", TimeZone: "UTC",
+		DisplayName: "Original Name", PreferredLanguage: "en", TimeZone: "UTC", VariantPreferences: []string{"erudit_ru"},
 	}); err != nil {
 		t.Fatalf("set name: %v", err)
 	}
@@ -50,7 +50,7 @@ func TestSeatNameFrozenAfterRename(t *testing.T) {
 
 	// Rename the account; the snapshot on the already-taken seat must not follow.
 	if _, err := accounts.UpdateProfile(ctx, starter, account.ProfileUpdate{
-		DisplayName: "Renamed Player99", PreferredLanguage: "en", TimeZone: "UTC",
+		DisplayName: "Renamed Player99", PreferredLanguage: "en", TimeZone: "UTC", VariantPreferences: []string{"erudit_ru"},
 	}); err != nil {
 		t.Fatalf("rename: %v", err)
 	}