Three patterns for control→cell silo communication

Control→cell silo communication patterns: (1) RPC services (preferred) — HMAC auth, e.g. project_service.get_by_id(). Control cannot query Organization, Project directly. (2) CellSiloClient — uses data= not body=, only works in CONTROL mode, throws ConnectionError. (3) HTTP self-call — requests against system.internal-url-prefix, works in MONOLITH. Gotcha: create_project_for_organization platform is str not optional.