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/src/main.rs

@@ -202,6 +202,8 @@ async fn wire_dependencies() -> anyhow::Result<(AppState, axum::Router)> {
         profile_fields_repository: profile_fields_repo,
         #[cfg(feature = "federation")]
         remote_watchlist_repository: remote_watchlist_repo,
+        #[cfg(feature = "federation")]
+        social_query: social_query.clone(),
         person_command,
         person_query,
         search_port,