Headline archive numbers for the public landing surface.

data_as_of is the newest LIVE forecast's own issued_at -- distinct from generated_at, which is only when THIS RESPONSE was built. A stalled forecaster and a healthy one answer identically on generated_at; data_as_of is what lets the ops watcher tell them apart. null on an empty archive, never today's date.

is_backtest = false is not optional here, the same way it is not optional for live_forecasts two lines up: a pastcast sweep writes backtest rows whose own issued_at can be newer than the live forecaster's last real run. Without the filter this field reports fresh from a sweep while the live forecaster is stalled -- the exact failure it exists to catch (caught in review by e0dad417 before deploy, not by this file's own test, which never seeded a backtest row; see test_stats_data_as_of_ignores_backtest_rows).

This is "newest live forecast issued", NOT the import-freshness stamp from #141 3c (imported_at/data_through). A monitor on this field detects a stalled forecaster, not a stalled push.