Add per ip cooldown to media.xiph.org captcha

I noticed this week on https://media.xiph.org/ the same ip address managed to download through the captcha the same 89G file 11 times in the same day. That's counting complete transfers, not the POST request or a handful of truncated or Range-request downloads.

If possible, I'd like to add a filter to block repeated downloads. Ideally one download per file per ip address, but something less specific would be helpful too. That seems to best way to limit waste like this without slowing down people who actually need multiple resources.