Add Prometheus metrics + Grafana panels for Overpass proxy

Adds four metrics surfaced via the existing /metrics endpoint:
- overpass_cache_events_total{result=hit|miss|coalesced}
- overpass_cache_size (gauge)
- overpass_upstream_duration_seconds (histogram)
- overpass_upstream_requests_total{status} — covers 2xx/4xx/5xx and
  an explicit "error" label for network-level failures

Grafana dashboards/planner.json gets a new row:
- Upstream health (5m 2xx success ratio, red <90%, green >99%)
- Cache hit ratio
- Cache size
- Upstream p95 latency
Plus timeseries panels for cache event breakdown, upstream status
over time, and p50/p95/p99 upstream latency.

The success-rate formula uses clamp_min(..., 1) on the denominator so
it doesn't NaN when traffic is zero.

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

infrastructure/grafana/dashboards/planner.json

@@ -97,6 +97,168 @@
           "legendFormat": "{{route}}"
         }
       ]
+    },
+    {
+      "title": "Overpass Upstream Health (5m success rate)",
+      "type": "stat",
+      "gridPos": {
+        "h": 5,
+        "w": 6,
+        "x": 0,
+        "y": 14
+      },
+      "targets": [
+        {
+          "expr": "sum(rate(overpass_upstream_requests_total{status=~\"2..\"}[5m])) / clamp_min(sum(rate(overpass_upstream_requests_total[5m])), 1)",
+          "legendFormat": "success"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "percentunit",
+          "min": 0,
+          "max": 1,
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              { "color": "red", "value": null },
+              { "color": "orange", "value": 0.9 },
+              { "color": "green", "value": 0.99 }
+            ]
+          }
+        }
+      }
+    },
+    {
+      "title": "Overpass Cache Hit Ratio (5m)",
+      "type": "stat",
+      "gridPos": {
+        "h": 5,
+        "w": 6,
+        "x": 6,
+        "y": 14
+      },
+      "targets": [
+        {
+          "expr": "sum(rate(overpass_cache_events_total{result=\"hit\"}[5m])) / clamp_min(sum(rate(overpass_cache_events_total[5m])), 1)",
+          "legendFormat": "hit ratio"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "percentunit",
+          "min": 0,
+          "max": 1
+        }
+      }
+    },
+    {
+      "title": "Overpass Cache Size",
+      "type": "stat",
+      "gridPos": {
+        "h": 5,
+        "w": 6,
+        "x": 12,
+        "y": 14
+      },
+      "targets": [
+        {
+          "expr": "overpass_cache_size",
+          "legendFormat": "entries"
+        }
+      ]
+    },
+    {
+      "title": "Overpass Upstream p95",
+      "type": "stat",
+      "gridPos": {
+        "h": 5,
+        "w": 6,
+        "x": 18,
+        "y": 14
+      },
+      "targets": [
+        {
+          "expr": "histogram_quantile(0.95, sum(rate(overpass_upstream_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p95"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "s"
+        }
+      }
+    },
+    {
+      "title": "Overpass Cache Events",
+      "type": "timeseries",
+      "gridPos": {
+        "h": 7,
+        "w": 12,
+        "x": 0,
+        "y": 19
+      },
+      "targets": [
+        {
+          "expr": "sum by (result) (rate(overpass_cache_events_total[5m]))",
+          "legendFormat": "{{result}}"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "ops"
+        }
+      }
+    },
+    {
+      "title": "Overpass Upstream Status",
+      "type": "timeseries",
+      "gridPos": {
+        "h": 7,
+        "w": 12,
+        "x": 12,
+        "y": 19
+      },
+      "targets": [
+        {
+          "expr": "sum by (status) (rate(overpass_upstream_requests_total[5m]))",
+          "legendFormat": "{{status}}"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "ops"
+        }
+      }
+    },
+    {
+      "title": "Overpass Upstream Latency",
+      "type": "timeseries",
+      "gridPos": {
+        "h": 7,
+        "w": 24,
+        "x": 0,
+        "y": 26
+      },
+      "targets": [
+        {
+          "expr": "histogram_quantile(0.50, sum(rate(overpass_upstream_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p50"
+        },
+        {
+          "expr": "histogram_quantile(0.95, sum(rate(overpass_upstream_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p95"
+        },
+        {
+          "expr": "histogram_quantile(0.99, sum(rate(overpass_upstream_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p99"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "s"
+        }
+      }
     }
   ]
 }