The data-freshness monitor classified by run RECENCY only, so the domrf_kn
FLATS loader running status=done but extracting 0 flats for ~5 weeks went
undetected — and the kn source watched objects_count (healthy ~1548), not
flats_count (the broken =0 metric).
Add an opt-in zero-output check: an otherwise-fresh run-ledger source (recent
success, would-be fresh by age) that produced 0 work-rows in the 7d window is
downgraded to status="failed" (so scrape_freshness_check alerts), with an
additive "reason". Guards: alert_on_zero_output flag, run-ledger only
(timestamp_col is None), status=="ok" (age-stale/failed already covered), and
upd_7d==0 (SUM of the source's own work_col over done-runs).
Registry: new kn_flats source (kn_scrape_runs, work_col=flats_count, critical,
flag on) — watches the column that was broken; existing kn (objects_count)
unchanged. Flag also enabled on objective (rows_lots, critical). nspd/nspd_geo/
cadastre left unflagged (legitimate-0 / data-table).
JSON additive only (new nullable "reason" key; endpoint is dict[str,Any], no
frontend consumer / no codegen needed). 4 new tests (downgrade, no-false-
positive, age-precedence, registry). code-reviewer APPROVE.
Would have caught #1945 within ~8-14d instead of 5 weeks.
