fix: address remaining 3 NOT DONE plan items

#18 featured collection: add featured_url to ApUser/DbActor/Person;
     serialized as featured field in AP JSON when set by consumer.

#19 Tombstone in Delete: broadcast_delete_to_followers now sends
     {"type":"Tombstone","id":"..."} instead of bare URL string.

#21 Backfill pagination: run_backfill uses get_local_objects_page
     with cursor-based loop — avoids loading all posts into memory;
     delivers newest-to-oldest in BATCH_SIZE chunks.

src/tests/actors.rs

@@ -34,6 +34,7 @@ fn person_serializes_with_enriched_fields() {
         image: None,
         also_known_as: vec![],
         attachment: vec![],
+        featured: Some("https://example.com/users/1/featured".parse().unwrap()),
     };
     let json = serde_json::to_value(&person).unwrap();
     assert_eq!(json["discoverable"], true);