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/handler/system_message_handler.go

@@ -42,6 +42,14 @@ func (h *SystemMessageHandler) GetSystemMessages(c *gin.Context) {
 		return
 	}
 
+	// 检查是否使用游标分页
+	_, cursorExists := c.GetQuery("cursor")
+	if cursorExists {
+		h.GetSystemMessagesByCursor(c)
+		return
+	}
+
+	// 偏移分页（向后兼容）
 	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
 	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
 
@@ -62,6 +70,43 @@ func (h *SystemMessageHandler) GetSystemMessages(c *gin.Context) {
 	response.Paginated(c, result, total, page, pageSize)
 }
 
+// GetSystemMessagesByCursor 游标分页获取系统消息列表
+func (h *SystemMessageHandler) GetSystemMessagesByCursor(c *gin.Context) {
+	userID := c.GetString("user_id")
+	if userID == "" {
+		response.Unauthorized(c, "")
+		return
+	}
+
+	// 解析游标分页请求
+	cursorStr := c.Query("cursor")
+	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
+
+	// 获取当前用户的系统通知
+	notifications, nextCursor, hasMore, err := h.notifyRepo.GetByReceiverIDCursor(userID, cursorStr, pageSize)
+	if err != nil {
+		response.InternalServerError(c, "failed to get system messages")
+		return
+	}
+
+	// 转换为响应格式
+	items := make([]*dto.SystemMessageResponse, 0)
+	for _, n := range notifications {
+		resp := dto.SystemNotificationToResponse(n)
+		items = append(items, resp)
+	}
+
+	// 构建游标分页响应
+	cursorResp := &dto.SystemMessageCursorPageResponse{
+		Items:      items,
+		NextCursor: nextCursor,
+		PrevCursor: "",
+		HasMore:    hasMore,
+	}
+
+	response.Success(c, cursorResp)
+}
+
 // GetUnreadCount 获取系统消息未读数
 // GET /api/v1/messages/system/unread-count
 func (h *SystemMessageHandler) GetUnreadCount(c *gin.Context) {