Discover all HOST*_TRANSCODE_SERVERS from every configured host.
For remote hosts, rewrite localhost/127.0.0.1 in the URL to their
Tailscale IP (via resolve_tailscale_ip). Unreachable hosts are
warned and skipped — partial sync continues for reachable servers.
Adding a new host needs no script changes — just HOST*_TRANSCODE_SERVERS
in the new host conf.