Add explicit UIDs to Grafana datasources

Alert rules reference datasourceUid: prometheus/loki which requires
explicit UIDs in the provisioning config.

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

infrastructure/grafana/provisioning/datasources/datasources.yml

@@ -2,6 +2,7 @@ apiVersion: 1
 
 datasources:
   - name: Prometheus
+    uid: prometheus
     type: prometheus
     access: proxy
     url: http://prometheus:9090
@@ -9,6 +10,7 @@ datasources:
     editable: false
 
   - name: Loki
+    uid: loki
     type: loki
     access: proxy
     url: http://loki:3100