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/user.rs

@@ -7,6 +7,24 @@ pub struct ApProfileField {
     pub value: String,
 }
 
+/// Resolved actor data returned by [`crate::service::ActivityPubService::lookup_actor_by_handle`].
+/// Fetched via a signed HTTP request so strict instances (e.g. Threads) return full data.
+#[derive(Debug, Clone)]
+pub struct LookedUpActor {
+    pub handle: String,
+    pub display_name: Option<String>,
+    pub bio: Option<String>,
+    pub avatar_url: Option<Url>,
+    pub banner_url: Option<Url>,
+    pub ap_url: Url,
+    pub outbox_url: Url,
+    pub followers_url: Url,
+    pub following_url: Url,
+    pub also_known_as: Option<String>,
+    pub profile_url: Option<Url>,
+    pub attachment: Vec<ApProfileField>,
+}
+
 #[derive(Debug, Clone)]
 pub struct ApUser {
     pub id: uuid::Uuid,