How fresh the public read copy is, from the stamp the hourly import swaps in WITH the data (r141 design §3c).

imported_at is when the copy being served was cut over: it stops moving when the push stops cutting over. data_through is the newest entity_metric_daily.day in that copy: it stops moving when upstream data stalls, whatever the push does. Both come from one row read in one statement.

Fail closed: if public_import_stamp is missing, empty, or has a null field, this answers 503 with a JSON detail.code -- never a 200 with a field missing or null, because an absent field reads as "fine" to a human and as "predates the stamp" to a monitor. Where no import has run (Halo, the lab, local dev) that is the correct answer: the freshness of a copy nobody imported is unknown. The monitor treats 404/503/garbage as INCONCLUSIVE (§3c.1 item 5), never as fresh.