E-invoices import - Add invoice ID to response

The POST /e-invoices/imports endpoint now returns the invoice id directly in the response body, making it easier to retrieve and track imported invoices.

Before

{
  "url": "https://app.pennylane.com/api/external/v2/customer_invoices/123"
}

After

{
  "id": "123",
  "url": "https://app.pennylane.com/api/external/v2/customer_invoices/123"
}

See the documentation: API Reference