Exposes host CPU, memory, disk, and network usage over HTTP for the
dashboard's System Stats block (or anything else that can send a header)
to poll. Auth is a required X-API-Key, auto-generated and persisted on
first run if not supplied, checked with a constant-time comparison and
rate-limited after repeated failures. No CORS, so a browser can't call it
directly from another origin -- only a server-side caller can, keeping
the key out of anyone's network tab.
