Without an explicit host, `server.listen(0)` binds to every interface,
turning safe-chain's unauthenticated forward proxy into an open relay
while `aikido-*` commands are running. Anyone reachable on the network
can use it to hit the victim's localhost, intranet, or cloud metadata
endpoints. The advertised HTTPS_PROXY URL already used `localhost`
(loopback), but the listener itself was wide open.
Bind to 127.0.0.1 explicitly and update the advertised URL to match.
Add a regression test that verifies the listener refuses connections
on non-loopback interfaces.
