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/domain/src/ports.rs

@@ -58,17 +58,31 @@ pub struct RemoteActorInfo {
     pub display_name: Option<String>,
 }
 
-/// New trait for social/federation read queries
+#[derive(Debug, Clone)]
+pub struct PendingFollowerInfo {
+    pub url: String,
+    pub handle: String,
+    pub display_name: Option<String>,
+    pub avatar_url: Option<String>,
+}
+
 #[async_trait]
 pub trait SocialQueryPort: Send + Sync {
-    /// Returns all accepted remote_actor_urls followed by `user_id`.
     async fn get_accepted_following_urls(
         &self,
         user_id: uuid::Uuid,
     ) -> Result<Vec<String>, DomainError>;
 
-    /// Returns all distinct remote actors followed by any local user on this instance.
     async fn list_all_followed_remote_actors(&self) -> Result<Vec<RemoteActorInfo>, DomainError>;
+
+    async fn count_following(&self, user_id: uuid::Uuid) -> Result<usize, DomainError>;
+
+    async fn count_accepted_followers(&self, user_id: uuid::Uuid) -> Result<usize, DomainError>;
+
+    async fn get_pending_followers(
+        &self,
+        user_id: uuid::Uuid,
+    ) -> Result<Vec<PendingFollowerInfo>, DomainError>;
 }
 
 #[async_trait]