Opaque service-health probe for this listing. Returns {"status": "ok"} when the service is healthy and {"status": "degraded"} when recent fetches have been failing at a high rate. Use this before invoking the data routes, but note that health is a single global in-process rolling window across all sources — not a per-source readiness signal. The response is intentionally opaque: it never exposes implementation details (proxies, impersonation, per-tier counts). A fresh Lambda is ok by default. This route does not consume the upstream fetch budget.