Favorite a hotel

Runs user.setFavorite with favorited: true. Send the hotel's hotelKey (hk_…, from search or detail) as entityId, plus its ref and the display fields name, imageUrl (https or site-relative), city, country, starRating (0-5) so the list can open and render the hotel on any device. A ref naming a different hotel than entityId is a 400. An sh_ ref sent as entityId is accepted and filed under its hotel key. Idempotent: favoriting something already favorited leaves one entry and refreshes its snapshot (unlike the web toggle). Returns { favorited: true, entityId }, entityId being the key the entry is stored under.