Update OpenAPI spec and clean up security classes

- Update OpenAPI spec to v0.3.2 with signature header documentation
- Add X-License-Signature and X-License-Timestamp header definitions
- Clean up unused imports in security classes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/SecureLicenseClient.php

@@ -17,7 +17,6 @@ use Psr\Cache\CacheItemPoolInterface;
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
-use Symfony\Contracts\HttpClient\ResponseInterface;
 
 /**
  * Secure license client with response signature verification.

src/Security/IntegrityChecker.php

@@ -4,8 +4,6 @@ declare(strict_types=1);
 
 namespace Magdev\WcLicensedProductClient\Security;
 
-use Magdev\WcLicensedProductClient\Exception\LicenseException;
-
 /**
  * Verifies the integrity of critical source files.
  *