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>

tests/Architecture/AuditModuleIsolationContractTest.php

@@ -17,9 +17,13 @@ final class AuditModuleIsolationContractTest extends TestCase
         ));
 
         $allowed = [
+            'ApiAuditServiceInterface.php',
+            'ApiSystemAuditReporterInterface.php',
             'AuditMetadataEnricherInterface.php',
             'AuditRecorderInterface.php',
             'ImportAuditInterface.php',
+            'NullApiAuditService.php',
+            'NullApiSystemAuditReporter.php',
             'NullAuditMetadataEnricher.php',
             'NullAuditRecorder.php',
             'NullImportAudit.php',