implement AuthenticateUser use case

input validation: email + password required. constructs
EmailAddress vo (BadRequest on bad format). looks up user; absent
or password-mismatch -> UnauthorizedException with constant
'invalid credentials' message (no enumeration leak). password
verified through PasswordHasher->verify against stored hash on
the User entity (no separate profile lookup -> tide keeps
password on the user row). returns the User entity for the
caller (typically CreateSession + AuthController). 27 tests
pass.

backend/app/Auth/UseCases/AuthenticateUser/AuthenticateUserRequest.php

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