Instead of completely hiding the Environment tab for non-owner/non-admin
users, show the variable keys with masked values (****) in a read-only
view. This lets members see which variables are configured without
exposing the actual values.
- Backend: mask values with "****" instead of nullifying custom_env
- Added custom_env_redacted boolean to API response
- Frontend: EnvTab supports readOnly mode with lock icon and muted styling