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/application/src/use_cases/mod.rs

@@ -9,6 +9,7 @@ pub mod enrich_movie;
 pub mod execute_import;
 pub mod export_diary;
 pub mod get_activity_feed;
+pub mod get_current_profile;
 pub mod get_diary;
 pub mod get_movie_social_page;
 pub mod get_movies;
@@ -20,11 +21,13 @@ pub mod get_review_history;
 pub mod get_user_profile;
 pub mod get_users;
 pub mod get_watchlist;
+pub mod get_watchlist_page;
 pub mod is_on_watchlist;
 pub mod list_import_profiles;
 pub mod log_review;
 pub mod login;
 pub mod register;
+pub mod register_and_login;
 pub mod remove_from_watchlist;
 pub mod save_import_profile;
 pub mod search;