refactor: extract business logic from handlers to application layer

Move domain logic out of 7 handlers into use cases:
- activity feed: FollowingFilter construction
- user profile: social counts + pending followers
- users list: parallel local+remote actor loading
- watchlist page: local-vs-remote branching
- sync_poster: movie lookup + validation
- get_profile: avatar URL construction
- post_register: register+login orchestration

Add SocialQueryPort.{count_following,count_accepted_followers,
get_pending_followers} to AppContext behind federation feature gate.

crates/presentation/tests/api_test.rs

@@ -370,6 +370,18 @@ impl domain::ports::SocialQueryPort for PanicSocialQuery {
     ) -> Result<Vec<domain::ports::RemoteActorInfo>, DomainError> {
         panic!()
     }
+    async fn count_following(&self, _: uuid::Uuid) -> Result<usize, DomainError> {
+        panic!()
+    }
+    async fn count_accepted_followers(&self, _: uuid::Uuid) -> Result<usize, DomainError> {
+        panic!()
+    }
+    async fn get_pending_followers(
+        &self,
+        _: uuid::Uuid,
+    ) -> Result<Vec<domain::ports::PendingFollowerInfo>, DomainError> {
+        panic!()
+    }
 }
 
 async fn test_app() -> Router {
@@ -402,6 +414,8 @@ async fn test_app() -> Router {
             profile_fields_repository: Arc::new(PanicProfileFields),
             #[cfg(feature = "federation")]
             remote_watchlist_repository: Arc::new(PanicRemoteWatchlist),
+            #[cfg(feature = "federation")]
+            social_query: Arc::new(PanicSocialQuery),
             person_command: Arc::new(PanicPersonCommand),
             person_query: Arc::new(PanicPersonQuery),
             search_port: Arc::new(PanicSearchPort),