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/worker/src/main.rs

@@ -60,7 +60,7 @@ async fn main() -> anyhow::Result<()> {
         fed_follow_repo,
         fed_actor_repo,
         fed_blocklist_repo,
-        _fed_social_query,
+        fed_social_query,
         fed_review_store,
         fed_remote_watchlist_repo,
     ) = match &db_pool {
@@ -91,6 +91,8 @@ async fn main() -> anyhow::Result<()> {
         profile_fields_repository: Arc::clone(&profile_fields_repo),
         #[cfg(feature = "federation")]
         remote_watchlist_repository: fed_remote_watchlist_repo.clone(),
+        #[cfg(feature = "federation")]
+        social_query: fed_social_query,
         person_command: Arc::clone(&person_command),
         person_query: Arc::clone(&person_query),
         search_port: Arc::clone(&search_port),