implement authenticate user use case

Green phase: AuthenticateUser validates credentials, throws BadRequestException for empty fields, UnauthorizedException for unknown email or wrong password.

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,
+    ) {}
+}