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/chat_test.go

@@ -19,10 +19,11 @@ const (
 )
 
 // chatAPI is a fake Bot API for the chat-gating tests: it answers getMe, returns a
-// scripted getChatMember status, and records restrictChatMember calls.
+// scripted getChatMember status, and records restrictChatMember and unpinChatMessage calls.
 type chatAPI struct {
 	memberStatus string // the status getChatMember reports (default "left")
 	restricts    []restrictCall
+	unpins       []unpinCall
 }
 
 type restrictCall struct {
@@ -30,6 +31,13 @@ type restrictCall struct {
 	canSend bool // can_send_messages in the applied permissions
 }
 
+// unpinCall records a single unpinChatMessage request (raw form values, mirroring the
+// restrictCall style).
+type unpinCall struct {
+	chatID    string
+	messageID string
+}
+
 func (a *chatAPI) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	switch {
 	case strings.HasSuffix(r.URL.Path, "/getMe"):
@@ -47,6 +55,9 @@ func (a *chatAPI) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		_ = json.Unmarshal([]byte(r.FormValue("permissions")), &perms)
 		a.restricts = append(a.restricts, restrictCall{userID: r.FormValue("user_id"), canSend: perms.CanSendMessages})
 		io.WriteString(w, `{"ok":true,"result":true}`)
+	case strings.HasSuffix(r.URL.Path, "/unpinChatMessage"):
+		a.unpins = append(a.unpins, unpinCall{chatID: r.FormValue("chat_id"), messageID: r.FormValue("message_id")})
+		io.WriteString(w, `{"ok":true,"result":true}`)
 	default:
 		io.WriteString(w, `{"ok":true,"result":true}`)
 	}
@@ -237,3 +248,44 @@ func TestApplyChatGateSkipsAbsentAndAdmin(t *testing.T) {
 		})
 	}
 }
+
+// autoForwardUpdate builds a message update as it lands in the discussion chat: a post in
+// chat chatID with id msgID and is_automatic_forward set to auto.
+func autoForwardUpdate(chatID int64, msgID int, auto bool) *models.Update {
+	return &models.Update{Message: &models.Message{
+		ID:                 msgID,
+		Chat:               models.Chat{ID: chatID, Type: models.ChatTypeSupergroup},
+		IsAutomaticForward: auto,
+	}}
+}
+
+func TestAutoForwardUnpinned(t *testing.T) {
+	// A channel post auto-forwarded into the moderated chat is unpinned by its exact id.
+	api := &chatAPI{}
+	b := newChatBot(t, api)
+	b.handleUpdate(context.Background(), b.api, autoForwardUpdate(testChatID, 42, true))
+	if len(api.unpins) != 1 || api.unpins[0].chatID != "555" || api.unpins[0].messageID != "42" {
+		t.Fatalf("unpins = %+v, want one {555, 42}", api.unpins)
+	}
+}
+
+func TestNonAutoForwardNotUnpinned(t *testing.T) {
+	// A normal message in the moderated chat (e.g. another admin's pinned message) is never
+	// unpinned — the is_automatic_forward filter is what guards every other pin.
+	api := &chatAPI{}
+	b := newChatBot(t, api)
+	b.handleUpdate(context.Background(), b.api, autoForwardUpdate(testChatID, 42, false))
+	if len(api.unpins) != 0 {
+		t.Fatalf("unpins = %+v, want none for a non-auto-forward message", api.unpins)
+	}
+}
+
+func TestAutoForwardOtherChatIgnored(t *testing.T) {
+	// An auto-forward in some other chat (not the configured moderated one) is ignored.
+	api := &chatAPI{}
+	b := newChatBot(t, api)
+	b.handleUpdate(context.Background(), b.api, autoForwardUpdate(999, 42, true))
+	if len(api.unpins) != 0 {
+		t.Fatalf("unpins = %+v, want none for a foreign chat", api.unpins)
+	}
+}