Get removed contacts for a list (tombstone feed)

Tombstone counterpart to GET /contacts?list_id=<id>. Returns contacts that have left the list — either dropped from it (ContactList.is_archived) or archived globally (Contact.is_archived) — so a sync consumer can reconcile removals incrementally via archived_since instead of diffing a full snapshot. is_archived distinguishes the two modes (gone globally vs only off this list). 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.