✨(helm) allow disallowing search engine indexing per instance

Add an optional ConfigMap-backed robots.txt mounted into the frontend
static export (frontend.robotsTxt.enabled/content), following the same
pattern as backend.themeCustomization, and enable it with a full
Disallow on the feature review-app environment.

src/helm/env.d/feature/values.impress.yaml.gotmpl

@@ -134,6 +134,12 @@ frontend:
 
   replicas: 1
 
+  robotsTxt:
+    enabled: true
+    content: |
+      User-agent: *
+      Disallow: /
+
   image:
     repository: lasuite/impress-frontend
     pullPolicy: Always