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,
+    ) {}
+}

backend/app/User/User.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+class User
+{
+    public function __construct(
+        private int $id,
+        private EmailAddress $email,
+        private string $passwordHash,
+        private bool $isAdmin,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getEmail(): EmailAddress
+    {
+        return $this->email;
+    }
+
+    public function getPasswordHash(): string
+    {
+        return $this->passwordHash;
+    }
+
+    public function isAdmin(): bool
+    {
+        return $this->isAdmin;
+    }
+}

backend/app/User/UserRepository.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+interface UserRepository
+{
+    public function create(CreateUserDto $dto): User;
+
+    public function find(int $id): ?User;
+
+    public function findByEmail(EmailAddress $email): ?User;
+}