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/flux-ui/flux-tailscale-service.yaml

@@ -10,9 +10,10 @@ spec:
   type: LoadBalancer
   loadBalancerClass: tailscale
   selector:
-    app.kubernetes.io/name: weave-gitops
+    app.kubernetes.io/name: gitops-server
+    app.kubernetes.io/instance: weave-gitops
   ports:
     - name: http
       port: 9001
       protocol: TCP
-      targetPort: 9001
+      targetPort: http