Validates go-api Authentik middleware against the production OIDC
provider at auth.lthn.io. Tests client_credentials token acquisition,
JWT validation via JWKS, forward-auth header extraction, RequireAuth/
RequireGroup enforcement, and permissive fail-open behaviour.
Gated behind AUTHENTIK_INTEGRATION=1 env var so normal CI is unaffected.
Also fixes README issuer URL to use auth.lthn.io.
Co-Authored-By: Virgil <virgil@lethean.io>
