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.

先閱讀未完成的 task matcher 和現有的 Quantum 工作設定。定義每週摘要如何根據使用者的技能選取排名前三的未指派開放任務，包括相符和不相符的技能以及建立日期。Done 也應涵蓋取消訂閱的處理；任務優先順序明確屬於可選的潤飾項目。

由索引模型根據 Issue 內容生成。