Readiness probe — Redis + per-scraper freshness

Returns Redis reachability + per-scraper freshness from scraper_runs.

Overall status is ok only when Redis is up AND every tracked scraper's latest successful run is within 2 * expected_interval_s. Otherwise degraded. Dead-man thresholds are read from scraper_runs rather than hardcoded constants (C1).

The value_staleness block (F1 per audit 2026-04-23) surfaces WGC per-currency mids that have stopped moving while the scraper keeps fetching. Per scope §3.2 Q2, entries in this block are INFORMATIONAL-ONLY — they do not flip overall status to degraded, because INR is currently upstream-stuck and coupling would leave /v1/status permanently degraded, drowning out other alerts. Monitoring consumers alert on the presence of entries directly.