Returns how current the dataset is, for callers that need to know whether a
query result can be trusted as up to date before acting on it.

latestEventAt is the commit time of the most recent event available to query, so
the lag against wall-clock time is the staleness of everything this API
returns. status is degraded when that lag exceeds the freshness target
while queries still succeed; a 503 means the dataset cannot be served at all.

No caller bearer token required, unlike every other endpoint. This service
authenticates to the node itself, using its own deployment-held service credential --
deliberately, since health checks are commonly hit by infrastructure (liveness/readiness
probes, monitoring) that has no real per-caller token to present. A failure to obtain or
use that credential is this deployment's own infrastructure problem, not the caller's, so
it surfaces as 503, not 401 -- there is no caller credential left for this endpoint to
reject.