add auth utility interfaces and impls

Clock + SystemClock (DateTimeImmutable in UTC), TokenGenerator +
RandomTokenGenerator (bin2hex(random_bytes(32)) -> 64-char hex),
PasswordHasher + BcryptPasswordHasher (password_hash with
PASSWORD_DEFAULT, password_verify). matching fakes:
FakeClock with mutable setTime, FakeTokenGenerator with a
pre-seeded queue (throws once exhausted), FakePasswordHasher
returns 'hashed:<plain>' for deterministic test assertions.
composer stan now passes --memory-limit=512M (default 128M
overflows once larastan loads more rules).

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;
+    }
+}