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/adapters/sqlite-federation/src/lib.rs

@@ -924,6 +924,64 @@ impl domain::ports::SocialQueryPort for SqliteFederationRepository {
             )
             .collect())
     }
+
+    async fn count_following(
+        &self,
+        user_id: uuid::Uuid,
+    ) -> Result<usize, domain::errors::DomainError> {
+        let uid = user_id.to_string();
+        let count: i64 = sqlx::query_scalar(
+            "SELECT COUNT(*) FROM ap_following WHERE local_user_id = ? AND status = 'accepted'",
+        )
+        .bind(&uid)
+        .fetch_one(&self.pool)
+        .await
+        .map_err(|e| domain::errors::DomainError::InfrastructureError(e.to_string()))?;
+        Ok(count as usize)
+    }
+
+    async fn count_accepted_followers(
+        &self,
+        user_id: uuid::Uuid,
+    ) -> Result<usize, domain::errors::DomainError> {
+        let uid = user_id.to_string();
+        let count: i64 = sqlx::query_scalar(
+            "SELECT COUNT(*) FROM ap_followers WHERE local_user_id = ? AND status = 'accepted'",
+        )
+        .bind(&uid)
+        .fetch_one(&self.pool)
+        .await
+        .map_err(|e| domain::errors::DomainError::InfrastructureError(e.to_string()))?;
+        Ok(count as usize)
+    }
+
+    async fn get_pending_followers(
+        &self,
+        user_id: uuid::Uuid,
+    ) -> Result<Vec<domain::ports::PendingFollowerInfo>, domain::errors::DomainError> {
+        let uid = user_id.to_string();
+        let rows = sqlx::query_as::<_, (String, String, Option<String>, Option<String>)>(
+            "SELECT ar.url, ar.handle, ar.display_name, ar.avatar_url
+             FROM ap_followers f
+             JOIN ap_remote_actors ar ON ar.url = f.remote_actor_url
+             WHERE f.local_user_id = ? AND f.status = 'pending'",
+        )
+        .bind(&uid)
+        .fetch_all(&self.pool)
+        .await
+        .map_err(|e| domain::errors::DomainError::InfrastructureError(e.to_string()))?;
+        Ok(rows
+            .into_iter()
+            .map(
+                |(url, handle, display_name, avatar_url)| domain::ports::PendingFollowerInfo {
+                    url,
+                    handle,
+                    display_name,
+                    avatar_url,
+                },
+            )
+            .collect())
+    }
 }
 
 #[async_trait]