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/app/Auth/TokenGenerator.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace App\Auth;
+
+interface TokenGenerator
+{
+    public function generate(): string;
+}