fix: return proper response for stale participant edit urls

src/Exception/ParticipantNotFoundException.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exception;
+
+use Symfony\Component\HttpKernel\Exception\HttpException;
+
+/**
+ * Exception thrown when attempting to access a participant that does not exist.
+ *
+ * This is used for stale or tampered participant index URLs in booking flows.
+ */
+class ParticipantNotFoundException extends HttpException
+{
+    public function __construct(int $index, ?\Throwable $previous = null)
+    {
+        $message = sprintf('Participant at index %d does not exist', $index);
+
+        parent::__construct(404, $message, $previous);
+    }
+}