Make sorting of invoice items reliable.

app/Http/Controllers/Api/InvoiceitemController.php

@@ -32,7 +32,7 @@ class InvoiceitemController extends Controller
             'name' => 'required|string',
             'description' => 'nullable|string',
             'article_number' => 'nullable|string',
-
+            'sort' => 'required|numeric|min:0',
         ]);
 
         $item = $invoice->items()->create($itemData);