fix: Correct pod selectors for Prometheus and Flux Tailscale services, use Doppler for Grafana creds

Prometheus needs operator.prometheus.io/name label selector. Flux UI pods
are labeled gitops-server not weave-gitops. Grafana now reads admin creds
from Doppler via ExternalSecret instead of hardcoded values.

infrastructure/addons/observability/kustomization.yaml

@@ -2,6 +2,7 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
   - namespace.yaml
+  - grafana-admin-externalsecret.yaml
   - helmrepository-prometheus-community.yaml
   - helmrepository-grafana.yaml
   - helmrelease-kube-prometheus-stack.yaml