feat(tg-bot): unpin auto-forwarded channel posts

Telegram non-disableably auto-pins each channel post it auto-forwards
into the linked discussion group. The bot now detects that message by
Message.is_automatic_forward in the moderated chat (TELEGRAM_CHAT_ID)
and unpins it by id, so a pin set by a human admin — or by the bot for
another message — is never touched (no unpinAllChatMessages).

Needs the can_pin_messages right in the chat; the startup self-check
now also warns when it is missing. Bot-only; no wire/schema/DB change.

platform/telegram/internal/bot/bot.go

@@ -285,6 +285,12 @@ func (t *Bot) logChatAdminStatus(ctx context.Context) {
 		return
 	}
 	t.log.Info("chat gating ready: bot is an admin with the restrict-members right", zap.Int64("chat_id", t.chatID))
+	// The same moderated chat also gets the linked channel's auto-forwarded posts un-pinned,
+	// which needs the pin-messages right; warn (separately from gating) when it is missing.
+	if !m.Administrator.CanPinMessages {
+		t.log.Warn("auto-unpin of linked channel posts WILL NOT WORK: the bot lacks the pin-messages right",
+			zap.Int64("chat_id", t.chatID))
+	}
 }
 
 // Notify sends a notification message with a Mini App launch button that opens the
@@ -405,6 +411,14 @@ func (t *Bot) handleUpdate(ctx context.Context, api *tgbot.Bot, update *models.U
 		t.handleSuccessfulPayment(ctx, update.Message)
 		return
 	}
+	// A channel post automatically forwarded into the moderated discussion chat is
+	// auto-pinned by Telegram (a non-disableable behaviour). Unpin that one message so only
+	// deliberate pins remain; it targets this exact message id, so a pin set by a human admin
+	// — or by the bot for another message — is never touched.
+	if m := update.Message; m != nil && m.IsAutomaticForward && t.chatID != 0 && m.Chat.ID == t.chatID {
+		t.handleLinkedChannelPin(ctx, m)
+		return
+	}
 	// Support relay (when enabled): a non-/start message — /start has its own handler —
 	// is either an operator's reply in the support chat or a user's direct message to
 	// relay. Everything else falls through to the Mini App launch reply.
@@ -421,6 +435,24 @@ func (t *Bot) handleUpdate(ctx context.Context, api *tgbot.Bot, update *models.U
 	t.handleStart(ctx, api, update)
 }
 
+// handleLinkedChannelPin removes Telegram's automatic pin from a linked channel's post
+// that was auto-forwarded into the moderated discussion chat. It unpins only that exact
+// message (by id), so a pin set by a human admin — or by the bot for another message — is
+// left untouched. It needs the bot to hold the pin-messages right in the chat; a missing
+// right surfaces as a warning (the unpin then no-ops), not a crash.
+func (t *Bot) handleLinkedChannelPin(ctx context.Context, m *models.Message) {
+	if _, err := t.api.UnpinChatMessage(ctx, &tgbot.UnpinChatMessageParams{
+		ChatID:    m.Chat.ID,
+		MessageID: m.ID,
+	}); err != nil {
+		t.log.Warn("could not unpin an auto-forwarded channel post; does the bot have the pin-messages right?",
+			zap.Int64("chat_id", m.Chat.ID), zap.Int("message_id", m.ID), zap.Error(err))
+		return
+	}
+	t.log.Debug("unpinned an auto-forwarded channel post",
+		zap.Int64("chat_id", m.Chat.ID), zap.Int("message_id", m.ID))
+}
+
 // SetEligibilityResolver wires the chat-eligibility resolver after construction (the
 // bot-link client backing it is built after the bot).
 func (t *Bot) SetEligibilityResolver(resolve EligibilityResolver) {