HttpUserEngineClient uses trusted-proxy claims against live user-engine.
Offline stub when URL/secret unset. Align default tenant with KeyCape
tenant:coulomb; map OIDC tenant/principal_type/groups into the envelope.
Public PKCE client on kc.coulomb.social with local and production redirect
URIs. Add register-keycape-client.sh, document env, and harden public-client
token exchange (no secret). Authorize probe verified registered vs reject.
