refactor: eliminate User/UserResponse struct literals, add AP user tests

- Feed/search adapters use #[sqlx(flatten)] UserRow instead of
  inline User construction — single point of change when User
  gains fields
- User::new_remote constructor replaces struct literal in testing
- to_summary_response replaces inline UserResponse in get_users
- 5 integration tests for PgApUserRepository (find, count,
  profile_fields→attachment)

crates/adapters/postgres-federation/migrations

@@ -0,0 +1 @@
+../postgres/migrations