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/bot/bot.go

@@ -191,6 +191,13 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	if update.Message == nil {
 		return
 	}
+	// Reply only in a private chat: the Mini App launch button is an inline web_app
+	// button, which Telegram permits only in private chats — replying to a group message
+	// (the bot is an admin in the moderated chat and now receives its messages) fails with
+	// BUTTON_TYPE_INVALID. In the group the bot only manages permissions, it never chats.
+	if update.Message.Chat.Type != models.ChatTypePrivate {
+		return
+	}
 	startParam := startPayload(update.Message.Text)
 	if _, err := api.SendMessage(ctx, &tgbot.SendMessageParams{
 		ChatID:      update.Message.Chat.ID,