Add stat card sparkline graphs with timeseries backend endpoint

New /search/stats/timeseries endpoint returns bucketed counts/metrics
over a time window using ClickHouse toStartOfInterval(). Frontend
Sparkline component renders SVG polyline + gradient fill on each
stat card, driven by a useStatsTimeseries query hook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/api/queries/executions.ts

@@ -28,6 +28,27 @@ export function useSearchExecutions(filters: SearchRequest) {
   });
 }
 
+export function useStatsTimeseries(timeFrom: string | undefined, timeTo: string | undefined) {
+  return useQuery({
+    queryKey: ['executions', 'timeseries', timeFrom, timeTo],
+    queryFn: async () => {
+      const { data, error } = await api.GET('/search/stats/timeseries', {
+        params: {
+          query: {
+            from: timeFrom!,
+            to: timeTo || undefined,
+            buckets: 24,
+          },
+        },
+      });
+      if (error) throw new Error('Failed to load timeseries');
+      return data!;
+    },
+    enabled: !!timeFrom,
+    refetchInterval: 30_000,
+  });
+}
+
 export function useExecutionDetail(executionId: string | null) {
   return useQuery({
     queryKey: ['executions', 'detail', executionId],