expires_at is stored as a database datetime column

### Context
During the DST to standard time change, when the SQL server is running in a local time zone, records cannot be found when `expires_at` is set to 1 hour.

Given a PST time and a fallback to standard time at 2 AM, 1 hour from 1 AM, is going to be still 1 AM. The date s converted to the same date that was entered and few seconds after the record cannot be found again.

The `expires_at` column should be timezone agnostic, possible solution would be to store it as an integer (epoch time),

从 expires_at 的模型或架构定义以及 ActiveRecord 的 datetime 转换入手，然后使用本地时区中的 SQL server 重现 DST fallback。完成的标准是：expiration 在整个 fallback 过程中仍然可查找，存储与时区无关，并且已就回归覆盖达成一致。

由索引模型根据 Issue 内容生成。