Maintain a spam email list to avoid sending

Maintain a list of emails that have reported spam to avoid sending again - with a function to explicitly opt back in. Can be queried by time, maybe by a campaign/custom id for attributing to email versions

The issue names no files, tests, or entry points. Start by inspecting the repository’s existing email-sending and persistence APIs, then clarify the spam-list, explicit opt-in, time-query, and campaign attribution requirements with maintainers. Done should match the agreed behavior and include coverage for it.

Written by the indexing model from the issue text.