Move failed count and avg duration from page-derived to backend stats

All stat card values now come from the /search/stats endpoint which
queries the full time window, not just the current page of results.
Consolidated into a single ClickHouse query for efficiency.

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

ui/src/pages/executions/ExecutionExplorer.tsx

@@ -24,12 +24,6 @@ export function ExecutionExplorer() {
   const total = data?.total ?? 0;
   const results = data?.data ?? [];
 
-  // Derive stats from current search results
-  const failedCount = results.filter((r) => r.status === 'FAILED').length;
-  const avgDuration = results.length > 0
-    ? Math.round(results.reduce((sum, r) => sum + r.durationMs, 0) / results.length)
-    : 0;
-
   const showFrom = total > 0 ? offset + 1 : 0;
   const showTo = Math.min(offset + limit, total);
 
@@ -50,8 +44,8 @@ export function ExecutionExplorer() {
       {/* Stats Bar */}
       <div className={styles.statsBar}>
         <StatCard label="Total Matches" value={total.toLocaleString()} accent="amber" change={`from current search`} sparkData={sparkTotal} />
-        <StatCard label="Avg Duration" value={`${avgDuration.toLocaleString()}ms`} accent="cyan" sparkData={sparkAvgDuration} />
-        <StatCard label="Failed (page)" value={failedCount.toLocaleString()} accent="rose" sparkData={sparkFailed} />
+        <StatCard label="Avg Duration" value={stats ? `${stats.avgDurationMs.toLocaleString()}ms` : '--'} accent="cyan" sparkData={sparkAvgDuration} />
+        <StatCard label="Failed" value={stats ? stats.failedCount.toLocaleString() : '--'} accent="rose" sparkData={sparkFailed} />
         <StatCard label="P99 Latency" value={stats ? `${stats.p99LatencyMs.toLocaleString()}ms` : '--'} accent="green" sparkData={sparkP99} />
         <StatCard label="In-Flight" value={stats ? stats.activeCount.toLocaleString() : '--'} accent="blue" change="running executions" sparkData={sparkActive} />
       </div>