Send an email digest with suggested tasks based on skills

# Problem

We want to send an email that uses the (yet unfinished) task matcher to send an email digest with perhaps the top three most recommended, unassigned, open tasks for a user based on their skills.

We can attempt to send this once a week using Quantum jobs.

We'll likely need to be add in some sort of system for being able to unsubscribe from these emails so that future emails don't accidentally end up in spam.

Each task should list:

- skills (both matching and not matching)
- when the task was created

Ideally we would also factor in the task priority, but that might be a polish step.

首先阅读未完成的 task matcher 和现有的 Quantum 作业设置。定义每周摘要如何根据用户的技能选择排名前三的未分配开放任务，包括匹配和不匹配的技能以及创建日期。Done 还应涵盖退订处理；任务优先级明确属于可选的润色项。

由索引模型根据 Issue 内容生成。