Fix error-rate-high alert firing on no data

The alert was firing with DatasourceNoData because zero 5xx responses
means the numerator returns empty (not 0). Fixed by:
- Adding `or vector(0)` so zero 5xx returns 0% instead of empty
- Setting `noDataState: OK` as a safety net

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

infrastructure/grafana/provisioning/alerting/alerts.yml

@@ -55,12 +55,13 @@ groups:
       - uid: error-rate-high
         title: Error rate > 5%
         condition: B
+        noDataState: OK
         data:
           - refId: A
             relativeTimeRange: { from: 600, to: 0 }
             datasourceUid: prometheus
             model:
-              expr: sum(rate(http_request_duration_seconds_count{status=~"5.."}[5m])) / clamp_min(sum(rate(http_request_duration_seconds_count[5m])), 0.001) * 100
+              expr: (sum(rate(http_request_duration_seconds_count{status=~"5.."}[5m])) or vector(0)) / clamp_min(sum(rate(http_request_duration_seconds_count[5m])), 0.001) * 100
               instant: true
           - refId: B
             datasourceUid: __expr__