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/api/schema.d.ts

@@ -217,6 +217,8 @@ export interface ProcessorNode {
 export type ProcessorSnapshot = Record<string, string>;
 
 export interface ExecutionStats {
+  failedCount: number;
+  avgDurationMs: number;
   p99LatencyMs: number;
   activeCount: number;
 }