refactor(audit): decouple API audit classes from core via interfaces

Core code (lib/) no longer imports any MintyPHP\Module\Audit\* classes directly.
New ApiAuditServiceInterface and ApiSystemAuditReporterInterface follow the
existing pattern (interface + null implementation in core, concrete binding
from module registrar). DoctorRunner and helpers/app.php now resolve through
DI interfaces, ensuring the audit module remains fully optional.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/Service/Audit/NullApiSystemAuditReporter.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace MintyPHP\Service\Audit;
+
+/**
+ * No-op API system audit reporter used when the audit module is disabled.
+ */
+final class NullApiSystemAuditReporter implements ApiSystemAuditReporterInterface
+{
+    public function start(): void
+    {
+    }
+
+    public function finish(int $statusCode, ?string $errorCode = null): void
+    {
+    }
+}