feat: correct handling of vouchers

src/BusProNet/Constants.php

@@ -48,6 +48,10 @@ final class Constants
     public const STATUS_BLOCKED = 'Buchungsstop';
     public const STATUS_ON_REQUEST = 'Anfrage';
 
+    // Booking types for API requests
+    public const BOOKING_TYPE_INQUIRY = 'Anfrage';
+    public const BOOKING_TYPE_BOOKING = 'Buchung';
+
     // Payment methods
     public const PAYMENT_METHOD_TRANSFER = 'transfer';
     public const PAYMENT_METHOD_DEBIT = 'debit';