docs: add inline documentation where missing

src/Twig/AppExtension.php

@@ -1,11 +1,20 @@
 <?php
 
+declare(strict_types=1);
+
 namespace App\Twig;
 
 use Twig\Extension\AbstractExtension;
 use Twig\TwigFilter;
 use Twig\TwigFunction;
 
+/**
+ * Twig extension registering booking-related filters and functions.
+ *
+ * Provides filters for formatting (file_size, format_money, format_service_price)
+ * and mapping (map_gender, map_status, map_country, map_nationality), plus
+ * functions for field state checking and participant eligibility.
+ */
 class AppExtension extends AbstractExtension
 {
     public function getFilters(): array