curl -X POST "https://api.nevatal.com/api/v1/billing/invoices/inv_002/approve" \
  -H "Authorization: Bearer nvtl_your_api_key"

{
  "id": "inv_002",
  "invoiceNumber": "FV-2025-0043",
  "status": "approved",
  "approvedAt": "2025-03-16T10:00:00Z"
}

{
  "statusCode": 422,
  "error": "Unprocessable Entity",
  "message": "Invoice is not in draft status"
}