Keep the email record by default

By default keep all emails and have a batch iterator & batch delete for manual handling.
Having a record of the last N days might be useful for debugging etc.

No files, tests, or entry points are named. Start by locating the component's email persistence and deletion paths, then clarify whether retention for the last N days is required and what batch iteration and deletion should expose. Done should mean the agreed default retention and manual batch handling are implemented and tested.

Written by the indexing model from the issue text.