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/user.rs

@@ -58,6 +58,9 @@ pub struct ApUser {
     pub manually_approves_followers: bool,
     /// AP actor type serialized in the actor JSON. Defaults to `Person`.
     pub actor_type: ApActorType,
+    /// URL of the `featured` (pinned posts) collection. Set to expose a pinned
+    /// posts collection in the actor JSON, compatible with Mastodon/Pleroma.
+    pub featured_url: Option<Url>,
 }
 
 #[async_trait]