"Awaiting" shows cases that are "approved" or "rejected" if submitter was notified

Travel request currently sends an email to the submitter when a case is approved or rejected. As a result, the submitter is "assigned" to the case in the "Approved" or "Rejected" state, so it shows up in the `/awaiting` endpoint. I think the resolution is for that query to exclude cases whose states have no actions.

从 `/awaiting` 端点及其 case 查询的实现开始。参考 issue 中描述的 approved 和 rejected 通知流程，检查 case 状态如何提供可用操作。完成标准是排除没有任何操作的 case，同时仍显示已分配且可执行操作的 case。

由索引模型根据 Issue 内容生成。