feat: signed actor lookup and display_name on DbActor

Add display_name field to DbActor, populated from AP Person.name in
from_json. Expose LookedUpActor type and lookup_actor_by_handle method
on ActivityPubService — uses the existing signed webfinger_https path
so strict instances (Threads, etc.) return full actor data.

src/lib.rs

@@ -25,4 +25,4 @@ pub use repository::{
     BlockedDomain, FederationRepository, Follower, FollowerStatus, FollowingStatus, RemoteActor,
 };
 pub use service::ActivityPubService;
-pub use user::{ApProfileField, ApUser, ApUserRepository};
+pub use user::{ApProfileField, ApUser, ApUserRepository, LookedUpActor};