Set TTL for challenges stored in redis

We are using auto-ssl to generate certs on the fly for custom domains owned by our customers. Once in a while someone sets a `CAA` record that prevents let's encrypt from issuing a cert. When this happens the challenge token in redis is not deleted and over time many hundreds can pile up. We are adding a check to the endpoint used in the `allow_domains` function to prevent this but in general it might be a good idea to set the TTL of challenge tokens to say 24 hours or at least expose a config parameter to do this.

Start by tracing where challenge tokens are stored in Redis and inspect the endpoint used by the allow_domains function. Confirm the intended 24-hour expiry behavior, then determine whether the TTL should be fixed or exposed as configuration; done means failed challenges no longer accumulate indefinitely.

Written by the indexing model from the issue text.