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>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/ExecutionStats.java

@@ -1,9 +1,11 @@
 package com.cameleer3.server.core.search;
 
 /**
- * Aggregate execution statistics.
+ * Aggregate execution statistics within a time window.
  *
- * @param p99LatencyMs 99th percentile duration in milliseconds within the requested time window
- * @param activeCount  number of currently running executions
+ * @param failedCount   number of failed executions
+ * @param avgDurationMs average duration in milliseconds
+ * @param p99LatencyMs  99th percentile duration in milliseconds
+ * @param activeCount   number of currently running executions
  */
-public record ExecutionStats(long p99LatencyMs, long activeCount) {}
+public record ExecutionStats(long failedCount, long avgDurationMs, long p99LatencyMs, long activeCount) {}