Feature: Scheduled actions

I don't think we need this for MVP, but many workflows will need something like this for practical use.
- When an indebtedness check is awaiting the submitter to "resubmit" info for more than 30 days, mark it "cancelled."
- When a FOIA request hasn't been fulfilled in 5 _business_ days, send an email requesting an extension
- When a contract has been in a step for more than 2 weeks, send a reminder to the assignee

Just leaving this issue here for when it's time.

该 issue 未指定任何文件、测试或入口点。首先梳理 workflows 如何表示等待状态和计划通知，然后明确提出的三个基于时间的行为，包括工作日处理方式和取消规则。Done 应定义为针对已达成一致的场景所实现的行为，并通过测试覆盖每条时间规则。

由索引模型根据 Issue 内容生成。