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/ports/noop.rs

@@ -41,7 +41,7 @@ pub struct NoopSocialCommand;
 
 #[async_trait]
 impl super::SocialCommand for NoopSocialCommand {
-    async fn follow(&self, _: &UserId, _: &SocialIdentity) -> Result<(), DomainError> {
+    async fn follow(&self, _: &UserId, _: &crate::value_objects::FollowTarget) -> Result<(), DomainError> {
         Ok(())
     }
     async fn unfollow(&self, _: &UserId, _: &SocialIdentity) -> Result<(), DomainError> {