feat(dashboard): windowed stats per range + clear-logs button

- Stats snapshot now computes the KPI totals AND the breakdown from the
  in-window per-minute buckets (previously they used all-time accumulators, so
  switching 1h/24h/7d changed only the chart, not the numbers). Buckets now
  carry failovers + per-group breakdown; Prometheus keeps all-time counters.
- New DELETE /admin/api/stats clears all in-memory stats and truncates the
  JSONL log; the dashboard gets a "Clear logs" button (with confirm).

web/js/api.js

@@ -87,6 +87,7 @@ export const api = {
 
   // stats
   stats: (range, groupby) => j("GET", `/stats?range=${range}&groupby=${groupby}`),
+  clearStats: () => j("DELETE", "/stats"),
   requests: (limit = 50, offset = 0) => j("GET", `/requests?limit=${limit}&offset=${offset}`),
 
   // playground