Many websites return an Etag in the http response header, indicating the version of the resource.  Could the etag (or a checksum of it) be recorded in the URL- key, the way size is now?  Then e.g. `fsck --from web` could do a stronger check that the same file is still downloadable from the web, and the situation where different remotes have different versions of a file with the same URL- key could be better prevented.
