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/account_test.go

@@ -159,46 +159,6 @@ func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 	}
 }
 
-// TestServiceLanguageRoundTrip checks SetServiceLanguage persists the push-routing
-// language (the bot a Telegram user last signed in through): a fresh account has
-// none, a set value reads back, a later login overwrites it (last-login-wins), and
-// an empty value is a no-op. The push-target route coalesces it with the preferred
-// language.
-func TestServiceLanguageRoundTrip(t *testing.T) {
-	ctx := context.Background()
-	store := account.NewStore(testDB)
-	acc, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
-	if err != nil {
-		t.Fatalf("provision telegram: %v", err)
-	}
-	if acc.ServiceLanguage != "" {
-		t.Errorf("fresh ServiceLanguage = %q, want empty", acc.ServiceLanguage)
-	}
-
-	if err := store.SetServiceLanguage(ctx, acc.ID, "ru"); err != nil {
-		t.Fatalf("set service language: %v", err)
-	}
-	if got, err := store.GetByID(ctx, acc.ID); err != nil {
-		t.Fatalf("get by id: %v", err)
-	} else if got.ServiceLanguage != "ru" {
-		t.Errorf("ServiceLanguage = %q, want ru", got.ServiceLanguage)
-	}
-
-	// A later login through the other bot updates it; a subsequent empty value
-	// (a non-Telegram login) leaves it unchanged.
-	if err := store.SetServiceLanguage(ctx, acc.ID, "en"); err != nil {
-		t.Fatalf("update service language: %v", err)
-	}
-	if err := store.SetServiceLanguage(ctx, acc.ID, ""); err != nil {
-		t.Fatalf("noop service language: %v", err)
-	}
-	if got, err := store.GetByID(ctx, acc.ID); err != nil {
-		t.Fatalf("get by id: %v", err)
-	} else if got.ServiceLanguage != "en" {
-		t.Errorf("ServiceLanguage after update+noop = %q, want en", got.ServiceLanguage)
-	}
-}
-
 // TestHighRateFlagRoundTrip covers the soft high-rate marker: a fresh account
 // is unflagged, FlagHighRate stamps it exactly once (a second sustained episode
 // never moves the timestamp), ClearHighRateFlag reverses it, and a re-flag after
@@ -299,6 +259,7 @@ func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 		PreferredLanguage:      "en",
 		TimeZone:               "UTC",
 		NotificationsInAppOnly: false,
+		VariantPreferences:     []string{"erudit_ru"},
 	})
 	if err != nil {
 		t.Fatalf("update profile: %v", err)