feat(monitoring): add API latency Grafana dashboard and alerting rules

Create comprehensive Grafana dashboard for API latency monitoring with:
- p50/p95/p99 stat panels and time series for all endpoints
- Per-endpoint latency breakdown with route/method template variables
- Top 10 slowest endpoints table and bar chart (by p99)
- Request rate (by method) and error rate (4xx/5xx) panels
- Error rate percentage (5xx/total) with SLO threshold
- Latency heatmap and histogram distribution panels

Add Prometheus alerting rules:
- ApiLatencyP99High: p99 > 1s for 5m (warning)
- ApiEndpointLatencyP99High: per-endpoint p99 > 2s (warning)
- ApiLatencyP99Critical: p99 > 3s for 3m (critical/SLO breach)
- ApiErrorRate5xxHigh: 5xx rate > 1% for 5m (warning)

Fix api-overview.json using wrong metric name
(http_request_duration_seconds → goodgo_api_request_duration_seconds).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

monitoring/grafana/dashboards/api-overview.json

@@ -48,17 +48,17 @@
       "gridPos": { "h": 8, "w": 12, "x": 0, "y": 8 },
       "targets": [
         {
-          "expr": "histogram_quantile(0.50, sum(rate(http_request_duration_seconds_bucket[5m])) by (le))",
+          "expr": "histogram_quantile(0.50, sum(rate(goodgo_api_request_duration_seconds_bucket[5m])) by (le))",
           "legendFormat": "p50",
           "refId": "A"
         },
         {
-          "expr": "histogram_quantile(0.95, sum(rate(http_request_duration_seconds_bucket[5m])) by (le))",
+          "expr": "histogram_quantile(0.95, sum(rate(goodgo_api_request_duration_seconds_bucket[5m])) by (le))",
           "legendFormat": "p95",
           "refId": "B"
         },
         {
-          "expr": "histogram_quantile(0.99, sum(rate(http_request_duration_seconds_bucket[5m])) by (le))",
+          "expr": "histogram_quantile(0.99, sum(rate(goodgo_api_request_duration_seconds_bucket[5m])) by (le))",
           "legendFormat": "p99",
           "refId": "C"
         }