For servers that can only reach the internet through a proxy: set
https_proxy, http_proxy and no_proxy in .env and Haven's outgoing requests
(link previews, the media proxy, bot callbacks, push notifications, update and
DDNS checks) go through it. Only http:// proxy addresses are supported, and
voice is unaffected.
With no proxy set, nothing changes. With one set, Haven warns at startup that
the proxy becomes part of its security: requests through it cannot be pinned
to the address Haven checked, so the proxy should be limited to the
destinations Haven needs. .env.example and GUIDE.md ("Outgoing Proxy") explain
it. Proxy passwords are hidden in the log, HTTPS certificates are checked
against the real target, and proxied requests time out and can be cancelled.