Perform asynchronous index creation

What does this MR do and why?

Ensures that the sent_notifications backfill is done, as well as schedules an asynchronous index creation for the id_convert_to_bigint column.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #389344 (closed)