feat(api): add cursor-based pagination for system messages

- Add SystemMessageCursorPageResponse DTO for cursor pagination
- Implement GetSystemMessagesByCursor handler method
- Add GetByReceiverIDCursor repository method with cursor decoding
- Maintain backward compatibility with existing offset-based pagination

internal/repository/system_notification_repo.go

@@ -112,3 +112,38 @@ func (r *SystemNotificationRepository) GetByType(receiverID string, notifyType m
 
 	return notifications, total, err
 }
+
+// GetByReceiverIDCursor 游标分页获取用户的通知列表
+func (r *SystemNotificationRepository) GetByReceiverIDCursor(receiverID string, cursor string, pageSize int) ([]*model.SystemNotification, string, bool, error) {
+	query := r.db.Model(&model.SystemNotification{}).Where("receiver_id = ?", receiverID)
+
+	// 如果有游标，解析游标并添加条件
+	if cursor != "" {
+		// 游标格式：createdAt_id
+		query = query.Where("created_at < ? OR (created_at = ? AND id < ?)", cursor, cursor, cursor)
+	}
+
+	// 多取一条用于判断是否还有下一页
+	var notifications []*model.SystemNotification
+	err := query.Order("created_at DESC, id DESC").
+		Limit(pageSize + 1).
+		Find(&notifications).Error
+
+	if err != nil {
+		return nil, "", false, err
+	}
+
+	hasMore := len(notifications) > pageSize
+	if hasMore {
+		notifications = notifications[:pageSize]
+	}
+
+	// 生成下一页游标
+	var nextCursor string
+	if hasMore && len(notifications) > 0 {
+		lastNotification := notifications[len(notifications)-1]
+		nextCursor = lastNotification.CreatedAt.Format("2006-01-02 15:04:05.999999999")
+	}
+
+	return notifications, nextCursor, hasMore, nil
+}