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),

expires_at のモデルまたはスキーマ定義と ActiveRecord の datetime 変換から始め、その後、ローカルタイムゾーンの SQL server で DST fallback を再現します。完了の条件は、fallback の前後を通じて expiration を検索でき、タイムゾーンに依存しないストレージと回帰テストのカバレッジについて合意できていることです。

索引モデルが issue の本文から書いたものです。