Previously we weren't setting the response date anywhere, it doesn't seem used but might as well return it just in case.
If an authentication request was approved already, we don't allow to do it again. This is to match with what Bitwarden is doing, though I don't think there's a way to actually do it or exploit it somehow.
Added an explicit time limit for the auth requests, previously we relied on our scheduled job to clean them up, and a user could technically disable that.