fix(grafana): repoint pipeline-performance trace panel to Tempo

The "Recent Pipeline Traces" panel still pointed to the removed
Jaeger datasource. Tempo's search API serves the same query shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infra/grafana/dashboards/pipeline-performance.json

@@ -148,7 +148,7 @@
       "title": "Recent Pipeline Traces",
       "type": "table",
       "gridPos": { "h": 8, "w": 24, "x": 0, "y": 14 },
-      "datasource": { "type": "jaeger", "uid": "jaeger" },
+      "datasource": { "type": "tempo", "uid": "tempo" },
       "targets": [
         {
           "query": "pipeline.run",