Fix stats query storm: stabilize time params to 10s granularity

PerformanceTab and RouteHeader computed new Date().toISOString() on every
render, producing unique millisecond timestamps that busted the React Query
cache key — causing continuous refetches (every few ms instead of 10s).
Round timestamps to 10-second boundaries with useMemo so the query key
stays stable between renders.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/routes/PerformanceTab.tsx

@@ -1,3 +1,4 @@
+import { useMemo } from 'react';
 import { useExecutionStats, useStatsTimeseries } from '../../api/queries/executions';
 import { StatCard } from '../../components/shared/StatCard';
 import { ThroughputChart } from '../../components/charts/ThroughputChart';
@@ -18,9 +19,16 @@ function pctChange(current: number, previous: number): { text: string; direction
   return { text: `${arrow} ${Math.abs(pct).toFixed(1)}% vs yesterday`, direction: pct > 0 ? 'up' : 'down' };
 }
 
+/** Round epoch-ms down to the nearest 10 s so the query key stays stable between renders. */
+function stableIso(epochMs: number): string {
+  return new Date(Math.floor(epochMs / 10_000) * 10_000).toISOString();
+}
+
 export function PerformanceTab({ group, routeId }: PerformanceTabProps) {
-  const timeFrom = new Date(Date.now() - 24 * 60 * 60 * 1000).toISOString();
-  const timeTo = new Date().toISOString();
+  const [timeFrom, timeTo] = useMemo(() => {
+    const now = Date.now();
+    return [stableIso(now - 24 * 60 * 60 * 1000), stableIso(now)];
+  }, [Math.floor(Date.now() / 10_000)]);
 
   // Use scoped stats/timeseries via group+routeId query params
   const { data: stats } = useExecutionStats(timeFrom, timeTo, routeId, group);

ui/src/pages/routes/RouteHeader.tsx

@@ -1,3 +1,4 @@
+import { useMemo } from 'react';
 import type { DiagramLayout } from '../../api/types';
 import { useExecutionStats } from '../../api/queries/executions';
 import styles from './RoutePage.module.css';
@@ -10,7 +11,10 @@ interface RouteHeaderProps {
 
 export function RouteHeader({ group, routeId, layout }: RouteHeaderProps) {
   const nodeCount = layout?.nodes?.length ?? 0;
-  const timeFrom = new Date(Date.now() - 24 * 60 * 60 * 1000).toISOString();
+  const timeFrom = useMemo(
+    () => new Date(Math.floor(Date.now() / 10_000) * 10_000 - 24 * 60 * 60 * 1000).toISOString(),
+    [Math.floor(Date.now() / 10_000)],
+  );
   const { data: stats } = useExecutionStats(timeFrom, undefined, routeId, group);
 
   const successRate = stats && stats.totalCount > 0