When the client of an image request goes away before its image is
ready, the image proxy returns context.Canceled. serveImage now logs
this once at info level, with the request_id, host and path, and
answers 499, the status nginx calls "client closed request". It was
answered 500 and logged at error level, as pixa's own failure. The
check sits in serveImage, before respondImageError, which cannot see
the request's context. A cancellation while the request's context is
live is still answered 500. README.md lists the 499 on both image
routes.
Model: opus-5-5
