PR #82 set TLS_FLAVOR=cert as the honest interim while the predecessor's
proxy owned /.well-known/acme-challenge outright. route-proxy took port
80 today and its handler passes unknown tokens through to routed paths
by design — the one line #82 promised, made now. The copied cert (valid
to Nov 27) stays on disk untouched; mailu's own certbot takes over from
here.