better handling payment purpose

app/Resources/InvoiceResource.php

@@ -30,6 +30,7 @@ class InvoiceResource {
         }
 
         $returnData['invoiceTypeShort'] = $this->invoice->invoiceType()->name;
+        $returnData['purpose'] = $this->invoice->purposeText();
         $returnData['costUnitName']  = $this->invoice->costUnit()->first()->name;
         $returnData['invoiceNumber'] = $this->invoice->invoice_number;
         $returnData['contactName'] = $this->invoice->contact_name;