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/context.rs

@@ -6,8 +6,8 @@ use domain::ports::{
     AuthService, DiaryExporter, DiaryRepository, DocumentParser, EventPublisher, ImageStorage,
     ImportProfileRepository, ImportSessionRepository, MetadataClient, MovieProfileRepository,
     MovieRepository, PasswordHasher, PersonCommand, PersonQuery, PosterFetcherClient,
-    ReviewRepository, SearchCommand, SearchPort, StatsRepository, UserProfileFieldsRepository,
-    UserRepository, WatchlistRepository,
+    ReviewRepository, SearchCommand, SearchPort, SocialQueryPort, StatsRepository,
+    UserProfileFieldsRepository, UserRepository, WatchlistRepository,
 };
 
 use crate::config::AppConfig;
@@ -38,5 +38,7 @@ pub struct AppContext {
     pub profile_fields_repository: Arc<dyn UserProfileFieldsRepository>,
     #[cfg(feature = "federation")]
     pub remote_watchlist_repository: Arc<dyn RemoteWatchlistRepository>,
+    #[cfg(feature = "federation")]
+    pub social_query: Arc<dyn SocialQueryPort>,
     pub config: AppConfig,
 }