BackendPlan and its standalone protobuf and installation endpoint have been
removed. The backend now stages and atomically activates one physical-plan
envelope containing the authoritative SummaryCatalog snapshot plus the
PrecomputePlan, optional CollectorPlan, TransmissionPlan, and QueryPlan
that reference catalog materialization IDs.
The current contracts and ownership rules are documented in
SummaryCatalog and SDS architecture.
The HTTP lifecycle is exposed through /api/v1/physical-plan,
/api/v1/physical-plan/activate, /api/v1/physical-plan/discard, and
/api/v1/physical-plan/status.