Generate a downloadable itinerary document

Generate a Word document version of the order's itinerary and return a signed download URL. The document is rendered in the site's default locale (not the caller's) so that operators printing for guests get the version the guest will read.

The download URL is short-lived; treat it as single-use and fetch a fresh one for each download. The document content matches the JSON itinerary returned by GET /shop/orders/{orderId}/itinerary.