add User entity, dto, repository interface

User holds email (EmailAddress vo), passwordHash, isAdmin - tide
keeps password and admin flag on the user row directly (no
separate profile entity like youngstartup). UserRepository
exposes find, findByEmail, create. CreateUserDto is readonly with
explicit isAdmin (per shared.md no-default-args rule).

backend/app/User/CreateUserDto.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+readonly class CreateUserDto
+{
+    public function __construct(
+        public EmailAddress $email,
+        public string $passwordHash,
+        public bool $isAdmin,
+    ) {}
+}