feat: account deletion request and active-user check

crates/application/tests/auth/register_test.rs

@@ -71,3 +71,34 @@ async fn registering_with_a_short_password_fails() {
         "expected password length error, got: {err}"
     );
 }
+
+#[tokio::test]
+async fn registration_refuses_a_retired_username() {
+    let users = InMemoryStore::new();
+    let b = TestContextBuilder::new().with_users(Arc::clone(&users) as _);
+    let deps = RegisterDeps {
+        user: b.user_repo.clone(),
+        password_hasher: b.password_hasher.clone(),
+        config: b.config.clone(),
+    };
+    register::execute(&deps, register_command("old@example.com", "ghost"))
+        .await
+        .unwrap();
+    let old = users
+        .find_by_email(&Email::new("old@example.com".into()).unwrap())
+        .await
+        .unwrap()
+        .unwrap();
+    users.mark_deleted(old.id(), &[]).await.unwrap();
+    // The purge removes the row; only the retired name keeps the username taken.
+    users.delete_user(old.id());
+
+    let err = register::execute(&deps, register_command("new@example.com", "Ghost"))
+        .await
+        .unwrap_err();
+
+    assert!(
+        matches!(&err, domain::errors::DomainError::ValidationError(m) if m == "Username already taken"),
+        "got {err:?}"
+    );
+}