fix: Use Flux valuesFrom to inject Doppler Grafana creds as Helm values

Switch from admin.existingSecret to valuesFrom so Flux reads the
Doppler-managed secret and injects credentials as standard Helm values.

infrastructure/addons/observability/helmrelease-kube-prometheus-stack.yaml

@@ -21,11 +21,18 @@ spec:
   upgrade:
     remediation:
       retries: 3
+  valuesFrom:
+    - kind: Secret
+      name: grafana-admin-credentials
+      valuesKey: admin-user
+      targetPath: grafana.admin.user
+    - kind: Secret
+      name: grafana-admin-credentials
+      valuesKey: admin-password
+      targetPath: grafana.admin.password
   values:
     grafana:
       enabled: true
-      admin:
-        existingSecret: grafana-admin-credentials
       grafana.ini:
         server:
           root_url: http://grafana.silverside-gopher.ts.net/