feat(application): add get_remote_friends use case

crates/application/src/use_cases/federation_management/mod.rs

@@ -96,6 +96,20 @@ pub async fn list_remote_following(
     federation.get_remote_following(user_id).await
 }
 
+pub async fn get_remote_friends(
+    federation: &dyn FederationActionPort,
+    user_id: &UserId,
+) -> Result<Vec<RemoteActor>, DomainError> {
+    use std::collections::HashSet;
+    let following = federation.get_remote_following(user_id).await?;
+    let followers = federation.get_remote_followers(user_id).await?;
+    let follower_urls: HashSet<&str> = followers.iter().map(|a| a.url.as_str()).collect();
+    Ok(following
+        .into_iter()
+        .filter(|a| follower_urls.contains(a.url.as_str()))
+        .collect())
+}
+
 pub async fn remove_remote_following(
     follows: &dyn FollowRepository,
     users: &dyn UserReader,

crates/application/src/use_cases/federation_management/tests.rs

@@ -1,6 +1,25 @@
 use super::*;
+use chrono::Utc;
+use domain::models::remote_actor::RemoteActor;
 use domain::testing::TestStore;
 
+fn remote_actor(url: &str, handle: &str) -> RemoteActor {
+    RemoteActor {
+        url: url.to_string(),
+        handle: handle.to_string(),
+        display_name: None,
+        avatar_url: None,
+        bio: None,
+        banner_url: None,
+        also_known_as: None,
+        outbox_url: None,
+        followers_url: None,
+        following_url: None,
+        attachment: vec![],
+        last_fetched_at: Utc::now(),
+    }
+}
+
 #[tokio::test]
 async fn list_pending_returns_empty_by_default() {
     let store = TestStore::default();
@@ -51,3 +70,41 @@ async fn list_remote_following_returns_empty_by_default() {
     let result = list_remote_following(&store, &uid).await.unwrap();
     assert!(result.is_empty());
 }
+
+#[tokio::test]
+async fn get_remote_friends_returns_intersection() {
+    let store = TestStore::default();
+    let uid = UserId::new();
+
+    let bob = remote_actor("https://bob.example.com/users/bob", "bob@bob.example.com");
+    let carol = remote_actor(
+        "https://carol.example.com/users/carol",
+        "carol@carol.example.com",
+    );
+
+    // uid follows bob and carol
+    store
+        .remote_following
+        .lock()
+        .unwrap()
+        .extend([bob.clone(), carol.clone()]);
+    // only bob follows back
+    store.remote_followers.lock().unwrap().push(bob.clone());
+
+    let friends = get_remote_friends(&store, &uid).await.unwrap();
+    assert_eq!(friends.len(), 1);
+    assert_eq!(friends[0].url, bob.url);
+}
+
+#[tokio::test]
+async fn get_remote_friends_empty_when_no_mutual() {
+    let store = TestStore::default();
+    let uid = UserId::new();
+
+    let bob = remote_actor("https://bob.example.com/users/bob", "bob@bob.example.com");
+    store.remote_following.lock().unwrap().push(bob.clone());
+    // bob does NOT follow back
+
+    let friends = get_remote_friends(&store, &uid).await.unwrap();
+    assert!(friends.is_empty());
+}