Get removed contacts for a newsletter (tombstone feed)

Returns contacts that were in the newsletter's audience but have since been removed — either archived globally (Contact.is_archived) or dropped from the newsletter's list(s) (ContactList.is_archived). Lets a sync consumer detect removals incrementally via archived_since instead of diffing a full snapshot. NOTE: archived_date is a proxy (Contact.updated / ContactList.updated — the row's last-modified time, which for an archival is the archival time) until a dedicated archived_at column exists, mirroring the v1 /persons/deleted feed.