Data-freshness health check

One row per product family: the latest backing-row timestamp and the number of rows written in the last 24h. Use it to health-check data freshness before acting on an endpoint. Each family reports its own ok flag; the overall response always succeeds. Where a family's rows can arrive without the value they exist for (crypto_etf: net_flow_usd), ok also requires the value to be filled for at least half the rows of the latest complete trading day, and the row carries value_column, value_day, value_rows, value_filled, value_coverage, value_ok and, when false, a reason.