Add HealthState and OpenUntil to adminProviderView, GET
/admin/api/overview's per-provider row. HealthState mirrors the
discovery circuit breaker's state: closed, open, or half-open.
OpenUntil is the zero time.Time unless the breaker is open.
A provider with discovery disabled, or one that has never failed a
refresh, always reads closed. This keeps the dashboard's default view
unchanged for a healthy deployment.
Both fields come from providerSnapshot, read in the same locked pass
as the existing lastRefresh/lastErr fields, so a concurrent
finishRefresh can never hand the dashboard a breaker state that
disagrees with the rest of the row.
