fix: FollowTarget enum — actor_url no longer lies about holding a handle

FollowCommand.target is now FollowTarget (Identity|Handle) instead of
SocialIdentity. actor_url field always holds a URL; handles go through
FollowTarget::Handle. Adapter resolves handles explicitly. Type system
prevents misuse in future commands.

crates/domain/src/tests/events.rs

@@ -1,5 +1,5 @@
 use super::*;
-use crate::value_objects::{SocialIdentity, UserId};
+use crate::value_objects::{FollowTarget, SocialIdentity, UserId};
 
 #[test]
 fn follow_accepted_matches() {
@@ -22,17 +22,33 @@ fn follow_accepted_matches() {
 }
 
 #[test]
-fn follow_requested_local() {
+fn follow_requested_with_identity() {
     let follower = UserId::from_uuid(uuid::Uuid::new_v4());
     let target = UserId::from_uuid(uuid::Uuid::new_v4());
     let event = DomainEvent::FollowRequested {
         follower: follower.clone(),
-        target: SocialIdentity::Local(target.clone()),
+        target: FollowTarget::Identity(SocialIdentity::Local(target.clone())),
     };
     assert!(matches!(
         event,
         DomainEvent::FollowRequested {
-            target: SocialIdentity::Local(_),
+            target: FollowTarget::Identity(SocialIdentity::Local(_)),
+            ..
+        }
+    ));
+}
+
+#[test]
+fn follow_requested_with_handle() {
+    let follower = UserId::from_uuid(uuid::Uuid::new_v4());
+    let event = DomainEvent::FollowRequested {
+        follower: follower.clone(),
+        target: FollowTarget::Handle("@alice@remote.example".into()),
+    };
+    assert!(matches!(
+        event,
+        DomainEvent::FollowRequested {
+            target: FollowTarget::Handle(_),
             ..
         }
     ));