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),

Start at the model or schema definition for expires_at and ActiveRecord's datetime conversion, then reproduce the DST fallback with the SQL server in a local timezone. Done means expiration remains findable across the fallback, with timezone-agnostic storage and regression coverage agreed.

Written by the indexing model from the issue text.