feat: voucher purchase flow

src/Exception/VoucherSessionNotFoundException.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exception;
+
+use Symfony\Component\HttpKernel\Exception\HttpException;
+
+/**
+ * Thrown when a voucher purchase step is reached without a running purchase, either directly or
+ * because the purchase was completed or the session expired.
+ */
+class VoucherSessionNotFoundException extends HttpException
+{
+    public function __construct(?\Throwable $previous = null)
+    {
+        parent::__construct(404, 'No valid voucher purchase session found. Please start a new purchase.', $previous);
+    }
+}