refactor(notification): migrate cursor pagination to use unified cursor package

- Add new GetByReceiverIDCursor method with context support and generic cursor types
- Rename old implementation to GetByReceiverIDCursorLegacy for backward compatibility
- Update handler to use legacy interface temporarily during migration
- Use CursorBuilder pattern for consistent pagination across services

internal/handler/system_message_handler.go

@@ -82,8 +82,8 @@ func (h *SystemMessageHandler) GetSystemMessagesByCursor(c *gin.Context) {
 	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)
+	// 获取当前用户的系统通知（使用兼容旧版接口）
+	notifications, nextCursor, hasMore, err := h.notifyRepo.GetByReceiverIDCursorLegacy(userID, cursorStr, pageSize)
 	if err != nil {
 		response.InternalServerError(c, "failed to get system messages")
 		return