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/events.rs

@@ -65,7 +65,7 @@ pub enum DomainEvent {
     },
     FollowRequested {
         follower: UserId,
-        target: crate::value_objects::SocialIdentity,
+        target: crate::value_objects::FollowTarget,
     },
     FollowAccepted {
         owner: UserId,