Configure non-fill period in health script

[compojoom]

Right now we treat a request as non-filled once the valid_until period is over. Since we increased the lock time significantly, we should have a way to issue a warning before the valid_until period expires giving agents time to react:
https://github.com/beamer-bridge/beamer/blob/main/beamer/health/check.py#L338

I think that the easiest way would be to have a env variable that we can set to specify how many minutes before valid_until expires we want to get notified.