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> {

crates/domain/src/ports/social.rs

@@ -7,14 +7,14 @@ use crate::{
         DiaryEntry, FederationFlags, RemoteActorInfo, RemoteGoalEntry, RemoteWatchlistEntry,
         WatchlistWithMovie,
     },
-    value_objects::{MovieId, SocialActor, SocialIdentity, UserId},
+    value_objects::{FollowTarget, MovieId, SocialActor, SocialIdentity, UserId},
 };
 
 // ── Unified social ports (ADR-0002) ─────────────────────────────────────────
 
 #[async_trait]
 pub trait SocialCommand: Send + Sync {
-    async fn follow(&self, follower: &UserId, target: &SocialIdentity) -> Result<(), DomainError>;
+    async fn follow(&self, follower: &UserId, target: &FollowTarget) -> Result<(), DomainError>;
 
     async fn unfollow(&self, follower: &UserId, target: &SocialIdentity)
     -> Result<(), DomainError>;