implement SignupUser use case

validates email present + format (wraps EmailAddress vo's
InvalidArgumentException as BadRequest), password present +
>= 8 chars, then ensures email not already registered. hashes
password through injected PasswordHasher and persists via
UserRepository->create with isAdmin=false (admins are seeder-
only per plan). throws DomainException on duplicate email so
the controller layer can map it to 409. all 18 tests pass.

backend/app/User/UseCases/SignupUser/SignupUserRequest.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace App\User\UseCases\SignupUser;
+
+class SignupUserRequest
+{
+    public function __construct(
+        public ?string $email,
+        public ?string $password,
+    ) {}
+}