chore: add php-cs-fixer with PSR-12 config and CI check

- Add friendsofphp/php-cs-fixer to require-dev
- Create .php-cs-fixer.dist.php configured for @PSR12 ruleset
- Add php-cs-fixer dry-run step to CI pipeline
- Auto-fix existing PSR-12 violations
- Document code style tooling in readme.md

tests/TestKernel.php

@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace App\Tests;
@@ -27,7 +28,7 @@ class TestKernel extends AppKernel
     {
         parent::build($container);
 
-        $container->addCompilerPass(new class implements CompilerPassInterface {
+        $container->addCompilerPass(new class () implements CompilerPassInterface {
             public function process(ContainerBuilder $container): void
             {
                 foreach (['nonceCache', 'rateLimitCache', 'sessionCache', 'sessionStorage'] as $poolId) {