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.

gateway/internal/transcode/transcode_telegram_test.go

@@ -43,11 +43,11 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
 		_ = json.NewDecoder(r.Body).Decode(&gotBody)
-		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван","service_language":"ru"}`))
+		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван"}`))
 	})
 	defer cleanup()
 
-	v := fakeValidator{user: connector.User{ExternalID: "42", Username: "neo", FirstName: "Иван", LanguageCode: "ru", ServiceLanguage: "ru", SupportedLanguages: []string{"ru"}}}
+	v := fakeValidator{user: connector.User{ExternalID: "42", Username: "neo", FirstName: "Иван", LanguageCode: "ru"}}
 	reg := transcode.NewRegistry(backend, v)
 	op, ok := reg.Lookup(transcode.MsgAuthTelegram)
 	if !ok {
@@ -62,48 +62,9 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 	if string(sess.Token()) != "tok-tg" || string(sess.UserId()) != "u-tg" {
 		t.Fatalf("session decoded wrong: token=%q user=%q", sess.Token(), sess.UserId())
 	}
-	// The validating bot's supported languages ride the Session so the UI gates the
-	// New Game variant choice (here: ru -> Russian + Эрудит only).
-	if got := sessionLanguages(sess); len(got) != 1 || got[0] != "ru" {
-		t.Errorf("session supported_languages = %v, want [ru]", got)
-	}
-	// The bot's service language rides the Session so the UI can build a share link to
-	// the same bot.
-	if got := string(sess.ServiceLanguage()); got != "ru" {
-		t.Errorf("session service_language = %q, want ru", got)
-	}
-	// The validated launch fields are forwarded so the backend can seed a new account;
-	// service_language is recorded to route the account's later out-of-app push.
-	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" || gotBody["service_language"] != "ru" {
-		t.Errorf("forwarded body = %+v, want external_id=42 language_code=ru first_name=Иван service_language=ru", gotBody)
-	}
-}
-
-// sessionLanguages reads the supported_languages vector off a decoded Session.
-func sessionLanguages(s *fb.Session) []string {
-	out := make([]string, s.SupportedLanguagesLength())
-	for i := range out {
-		out[i] = string(s.SupportedLanguages(i))
-	}
-	return out
-}
-
-// TestGuestAuthSeedsDefaultLanguages confirms a non-platform (guest) session carries
-// the gateway's default supported-languages set, so the web client is ungated.
-func TestGuestAuthSeedsDefaultLanguages(t *testing.T) {
-	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, _ *http.Request) {
-		_, _ = w.Write([]byte(`{"token":"tok-g","user_id":"u-g","is_guest":true,"display_name":"Guest"}`))
-	})
-	defer cleanup()
-
-	reg := transcode.NewRegistry(backend, nil, "en", "ru")
-	op, _ := reg.Lookup(transcode.MsgAuthGuest)
-	payload, err := op.Handler(context.Background(), transcode.Request{})
-	if err != nil {
-		t.Fatalf("handler: %v", err)
-	}
-	if got := sessionLanguages(fb.GetRootAsSession(payload, 0)); len(got) != 2 || got[0] != "en" || got[1] != "ru" {
-		t.Errorf("guest session supported_languages = %v, want [en ru]", got)
+	// The validated launch fields are forwarded so the backend can seed a new account.
+	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" {
+		t.Errorf("forwarded body = %+v, want external_id=42 language_code=ru first_name=Иван", gotBody)
 	}
 }