Public accuracy ledger — source-health summary + change-observation stats

The machine-readable form of the public /accuracy page. Returns the latest source-liveness sweep summary (how many cited sources were checked, verified live, confirmed broken, or unverified), the count of confirmed-broken sources in the review queue, and the total append-only change-observations on record. Free/public: no tier gate, rate-limited like any endpoint. Every figure is a live count over real rows.