feat: fold purchase and promo voucher fields into one code field

src/Enum/VoucherCodeKind.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enum;
+
+/**
+ * What a code typed into the booking voucher field is, as far as its format tells.
+ *
+ * Kauf vs. Kulanz is not visible in the code; BusPro tells them apart when the purchase code is checked.
+ */
+enum VoucherCodeKind: string
+{
+    // Aktionsgutschein, checked with AKTIONSGUTSCHEIN and sent as the participant's <aktionscode>
+    case Promo = 'promo';
+
+    // Kauf- or Kulanzgutschein, checked with GUTSCHEINPRUEFUNGEINLOESUNG
+    case Purchase = 'purchase';
+
+    // Cannot be either, so it is rejected without asking BusPro
+    case Malformed = 'malformed';
+}