fix(telegram): reply to /start only in private chats

The main bot is now an admin in the moderated discussion group and receives its
messages (allowed_updates includes message). Its default handler replied to
every message with a Mini App launch button — an inline web_app button, which
Telegram permits only in private chats — so replying in the group failed with
BUTTON_TYPE_INVALID (silently: the send fails, no user-facing error). Reply only
in a private chat; in the group the bot only manages permissions. The promo bot
gets the same guard.

platform/telegram/internal/promobot/promobot.go

@@ -92,6 +92,11 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	if update.Message == nil {
 		return
 	}
+	// Only respond to a private /start: the promo bot is a one-on-one onboarding entry
+	// point and should never reply to group messages.
+	if update.Message.Chat.Type != models.ChatTypePrivate {
+		return
+	}
 	if err := t.throttle(ctx); err != nil {
 		return
 	}

platform/telegram/internal/promobot/promobot_test.go

@@ -85,7 +85,7 @@ func TestHandleStartReplies(t *testing.T) {
 	}
 
 	b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
-		Chat: models.Chat{ID: 42},
+		Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate},
 		From: &models.User{LanguageCode: "ru"},
 		Text: "/start f99",
 	}})
@@ -103,3 +103,19 @@ func TestHandleStartReplies(t *testing.T) {
 		t.Errorf("reply_markup = %q, want startapp=f99 (the /start payload forwarded)", api.replyMarkup)
 	}
 }
+
+func TestHandleStartIgnoresGroup(t *testing.T) {
+	api := &fakeAPI{}
+	srv := httptest.NewServer(api)
+	t.Cleanup(srv.Close)
+	b, err := New(Config{Token: "1:2", APIBaseURL: srv.URL, BotUsername: "B", BotLinkURL: "https://t.me/b/a"}, zap.NewNop())
+	if err != nil {
+		t.Fatalf("new: %v", err)
+	}
+	b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+		Chat: models.Chat{ID: -100, Type: models.ChatTypeSupergroup}, Text: "/start",
+	}})
+	if api.chatID != "" {
+		t.Errorf("replied to a group message (chat=%q); want none", api.chatID)
+	}
+}