HL node ops snapshot

Proxies `GET /admin/ops-status` on the HL VM (hl-adapter). Returns systemd unit status, disk usage, memory, load avg, and Caddy LE cert info — all read-only. If `art.hl.admin_url` / `art.hl.admin_token` is unset, returns `{available: false, reason: ...}` instead of an HTTP error so the UI can render a configuration hint.