Fix error rate alert: avoid NaN when no traffic

The error rate formula (errors/total*100) produces NaN when there's
zero traffic (0/0). Grafana treats NaN as a firing condition.
Use clamp_min on the denominator to return 0% instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infrastructure/grafana/provisioning/alerting/alerts.yml

@@ -60,7 +60,7 @@ groups:
             relativeTimeRange: { from: 600, to: 0 }
             datasourceUid: prometheus
             model:
-              expr: sum(rate(http_request_duration_seconds_count{status=~"5.."}[5m])) / sum(rate(http_request_duration_seconds_count[5m])) * 100
+              expr: sum(rate(http_request_duration_seconds_count{status=~"5.."}[5m])) / clamp_min(sum(rate(http_request_duration_seconds_count[5m])), 0.001) * 100
               instant: true
           - refId: B
             datasourceUid: __expr__