Redirect to a media asset at a given width

Answers 302 to a resized copy at the given width, falling back to the original when none exists yet, cached for an hour.

Same redirect as GET /v1/public/media/{id}, at one of the widths Sorai generates for responsive images. A width outside that set is rejected outright; a media asset that predates derivative generation, or is already narrower than the requested width, redirects to the original instead of erroring.