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/service/broadcast.rs

@@ -119,7 +119,7 @@ impl ActivityPubService {
             id: activity_url(&self.base_url).map_err(|e| anyhow::anyhow!("{e}"))?,
             kind: Default::default(),
             actor: ObjectId::from(local_actor.ap_id.clone()),
-            object: serde_json::json!(ap_id.to_string()),
+            object: serde_json::json!({"type": "Tombstone", "id": ap_id.to_string()}),
             to: vec![crate::urls::AS_PUBLIC.to_string()],
             cc: vec![local_actor.followers_url.to_string()],
         };