Fix blueprint !Env tag: use scalar not sequence

!Env expects a bare string (e.g. !Env FOO), not a YAML sequence
(!Env [FOO]). The list form caused IndexError during blueprint
discovery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

argocd/manifests/authentik/configmap-blueprint.yaml

@@ -31,7 +31,7 @@ data:
           invalidation_flow: !Find [authentik_flows.flow, [slug, default-provider-invalidation-flow]]
           client_type: confidential
           client_id: grafana
-          client_secret: !Env [AUTHENTIK_GRAFANA_CLIENT_SECRET]
+          client_secret: !Env AUTHENTIK_GRAFANA_CLIENT_SECRET
           redirect_uris:
             - matching_mode: strict
               url: https://grafana.ops.eblu.me/login/generic_oauth