Wire user-engine HTTP /me for member provisioning (CSOC-WP-0002-T04)

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.

coulomb_social/apps/core/views.py

@@ -32,6 +32,7 @@ def app_home(request: HttpRequest) -> HttpResponse:
                     str(member.user_engine_user_id) if member else ""
                 ),
                 "tenant_id": member.tenant_id if member else "",
+                "user_engine_source": request.session.get("user_engine_source", ""),
             },
         },
     )