fix(grafana): Use existing secret for admin credentials

Switch to admin.existingSecret to avoid rendering the admin password
into git. The secret must be created manually in the cluster.

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

prototypes/grafana/helm/grafana.yaml

@@ -1,4 +1,7 @@
 ---
+admin:
+  existingSecret: grafana-admin
+
 initChownData:
   enabled: false
 

rendered/envs/production/grafana/deployment-grafana.yaml

@@ -25,7 +25,6 @@ spec:
         checksum/config: de8d6f16e9721409f5848bcc101e6aa9815e6455bd4fb9b59306159e705ac1cb
         checksum/dashboards-json-config: 63ff5f7bd5ab0b6c241c689c0aa4d78be9bef984e63c1089dc988905fbb61f74
         checksum/sc-dashboard-provider-config: e70bf6a851099d385178a76de9757bb0bef8299da6d8443602590e44f05fdf24
-        checksum/secret: 8f37444562de973fdc7cb492e33bb12625463982ca068e8b65c28bc073359926
         kubectl.kubernetes.io/default-container: grafana
       labels:
         app.kubernetes.io/instance: grafana
@@ -44,12 +43,12 @@ spec:
               valueFrom:
                 secretKeyRef:
                   key: admin-user
-                  name: grafana
+                  name: grafana-admin
             - name: GF_SECURITY_ADMIN_PASSWORD
               valueFrom:
                 secretKeyRef:
                   key: admin-password
-                  name: grafana
+                  name: grafana-admin
             - name: GF_PATHS_DATA
               value: /var/lib/grafana/
             - name: GF_PATHS_LOGS