docs: add inline documentation where missing

src/Validator/Constraints/Participant.php

@@ -1,9 +1,17 @@
 <?php
 
+declare(strict_types=1);
+
 namespace App\Validator\Constraints;
 
 use Symfony\Component\Validator\Constraint;
 
+/**
+ * Validates participant data for cross-field consistency.
+ *
+ * Applied as a class-level constraint on ParticipantDto to enforce business rules
+ * that depend on multiple fields (e.g., requiring pickup when bus transport is selected).
+ */
 #[\Attribute]
 class Participant extends Constraint
 {