fix(chat): 撤回消息时回退未读角标，避免当成新消息

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/chat.go

@@ -918,30 +918,31 @@ func (s *ChatService) ListMessages(userID, roomID, beforeID uint, size int, over
 	return msgs, hasMore, nil
 }
 
-// RecallMessage 撤回消息：群主可撤自己的；监管者可撤任意；标记 RecalledAt，正文保留供审计
-func (s *ChatService) RecallMessage(operatorID, roomID, messageID uint, oversee bool) (*model.ChatMessage, error) {
+// RecallMessage 撤回消息：群主可撤自己的；监管者可撤任意；标记 RecalledAt，正文保留供审计。
+// newly=true 表示本次刚打上撤回标记（已撤回的幂等调用为 false，避免重复回退角标）。
+func (s *ChatService) RecallMessage(operatorID, roomID, messageID uint, oversee bool) (*model.ChatMessage, bool, error) {
 	if _, err := s.getRoom(roomID); err != nil {
-		return nil, err
+		return nil, false, err
 	}
 	var msg model.ChatMessage
 	if err := s.db.Where("id = ? AND room_id = ?", messageID, roomID).First(&msg).Error; err != nil {
 		if errors.Is(err, gorm.ErrRecordNotFound) {
-			return nil, ErrChatMsgNotFound
+			return nil, false, ErrChatMsgNotFound
 		}
-		return nil, err
+		return nil, false, err
 	}
 	if msg.RecalledAt != nil {
-		return &msg, nil // 幂等
+		return &msg, false, nil // 幂等
 	}
 	if !oversee {
 		m, err := s.membership(s.db, roomID, operatorID)
 		if err != nil {
-			return nil, err
+			return nil, false, err
 		}
 		// 群主/成员仅可撤回自己的消息；监管者可撤任意
 		if msg.SenderID != operatorID {
 			_ = m // 成员校验已通过
-			return nil, ErrChatRecallDenied
+			return nil, false, ErrChatRecallDenied
 		}
 	}
 	now := time.Now()
@@ -950,12 +951,12 @@ func (s *ChatService) RecallMessage(operatorID, roomID, messageID uint, oversee
 		"recalled_at": now,
 		"recalled_by": operatorID,
 	}).Error; err != nil {
-		return nil, err
+		return nil, false, err
 	}
 	msg.RecalledAt = &now
 	msg.RecalledBy = operatorID
 	_ = s.db.Preload("Sender").Preload("Recaller").First(&msg, msg.ID)
-	return &msg, nil
+	return &msg, true, nil
 }
 
 // AdminChatParticipant 管理端私聊双方摘要