test: add failing authenticate user test

Red phase: write AuthenticateUserTest with cases for valid credentials, empty email/password (null and empty string), unknown email, wrong password, and fresh instance guarantee. Fakes included.

backend/tests/Fakes/FakeClock.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Tests\Fakes;
+
+use App\Auth\Clock;
+use DateTimeImmutable;
+
+class FakeClock implements Clock
+{
+    public function __construct(private DateTimeImmutable $currentTime) {}
+
+    public function now(): DateTimeImmutable
+    {
+        return $this->currentTime;
+    }
+
+    public function setTime(DateTimeImmutable $newTime): void
+    {
+        $this->currentTime = $newTime;
+    }
+}