Add Nominatim lookup duration to Grafana dashboard

- Added query for fingr_nominatim_lookup_duration_seconds to Processing Time Breakdown panel
- Shows p95 latency of Nominatim geocoding lookups in stacked graph
- Helps identify when external geocoding service becomes a bottleneck

Author: Agent

etc/grafana/provisioning/dashboards/fingr-dashboard.json

@@ -743,6 +743,15 @@
           "expr": "histogram_quantile(0.95, rate(fingr_formatting_duration_seconds_bucket[5m]))",
           "legendFormat": "Formatting",
           "refId": "C"
+        },
+        {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "prometheus"
+          },
+          "expr": "histogram_quantile(0.95, rate(fingr_nominatim_lookup_duration_seconds_bucket[5m]))",
+          "legendFormat": "Nominatim Lookup",
+          "refId": "D"
         }
       ],
       "title": "Processing Time Breakdown (p95)",