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/openapi.json

@@ -1049,6 +1049,14 @@
       "ExecutionStats": {
         "type": "object",
         "properties": {
+          "failedCount": {
+            "type": "integer",
+            "format": "int64"
+          },
+          "avgDurationMs": {
+            "type": "integer",
+            "format": "int64"
+          },
           "p99LatencyMs": {
             "type": "integer",
             "format": "int64"