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/test_helpers.rs

@@ -2,6 +2,8 @@ use std::sync::Arc;
 
 #[cfg(feature = "federation")]
 use domain::testing::PanicRemoteWatchlistRepository;
+#[cfg(feature = "federation")]
+use domain::testing::PanicSocialQueryPort;
 use domain::{
     ports::{
         AuthService, DiaryExporter, DiaryRepository, DocumentParser, EventPublisher, ImageStorage,
@@ -144,6 +146,8 @@ impl TestContextBuilder {
             config: self.config,
             #[cfg(feature = "federation")]
             remote_watchlist_repository: std::sync::Arc::new(PanicRemoteWatchlistRepository),
+            #[cfg(feature = "federation")]
+            social_query: std::sync::Arc::new(PanicSocialQueryPort),
         }
     }
 }