Content-type fixer for thrift listing photos. ShopGoodwill's CDN serves images as
application/octet-stream, so Slack renders them as download links instead of pictures.
This re-serves the same public bytes with a correct image/* type.
Usage: /img?u=<url-encoded image url>
Only a fixed allowlist of public thrift-site CDN hosts can be fetched — it is not an open proxy.