How the Program Works Together

1. Client sends a POST request to /api/invoices/generate with the invoice data in JSON format.
2. The request data is validated. If invalid, an error message is returned.
3. If the request is valid:
   • The HTML template is loaded and populated with the invoice data.
   • The populated template is converted to a PDF using iText.
   • The PDF is returned as a file in the HTTP response.