Fixes#306 — OIDC scopes were hardcoded to 'openid email profile',
causing OIDC_ADMIN_CLAIM-based role mapping to fail when the required
scope (e.g. 'groups') wasn't requested. The new OIDC_SCOPE variable
defaults to 'openid email profile groups' so group-based admin mapping
works out of the box. Variable is now documented in README, docker-compose,
.env.example, and the Helm chart values.