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-search/src/tests.rs

@@ -5,6 +5,7 @@ use domain::{
         user::User,
     },
     ports::{SearchPort, ThoughtRepository, UserWriter},
+    value_objects::{Content, Email, PasswordHash, ThoughtId, UserId, Username},
 };
 
 async fn seed_thought(pool: &sqlx::PgPool, username: &str, content: &str) -> (User, Thought) {