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/i18n/en.js

@@ -59,6 +59,9 @@ export default {
   // dashboard
   "dash.title": "Dashboard",
   "dash.subtitle": "Live traffic, cost and routing analytics for your model pool.",
+  "dash.clear": "Clear logs",
+  "dash.clearConfirm": "Clear all recorded stats and logs? This cannot be undone.",
+  "dash.cleared": "Logs cleared",
   "dash.range": "Range",
   "dash.range.1h": "1h",
   "dash.range.24h": "24h",

web/js/i18n/ru.js

@@ -58,6 +58,9 @@ export default {
   // dashboard
   "dash.title": "Дашборд",
   "dash.subtitle": "Трафик в реальном времени, расходы и аналитика роутинга по пулу моделей.",
+  "dash.clear": "Очистить логи",
+  "dash.clearConfirm": "Очистить всю статистику и логи? Отменить нельзя.",
+  "dash.cleared": "Логи очищены",
   "dash.range": "Период",
   "dash.range.1h": "1ч",
   "dash.range.24h": "24ч",