Per-event price volatility over the trailing 24h

Aggregates `event_metrics_5m` over the trailing 24h to surface how stable the cross-provider YES mid has been. Overall stats span (provider × time); `byProvider` filters to one provider; `series` is a per-5m-bucket cross-provider mean + stddev for charting a band around the mean. Cached for 30s per universalId. The 5m bucket cadence is the upstream rollup granularity — a fresh recompute cannot surface data older than the most recent bucket.