Topology rows deleted without a proper teardown leave Docker containers
and bridge networks behind, holding IPAM pools that cause 403 "Pool
overlaps" on the next deploy at the same subnet.
- engine/reaper.py walks the local Docker daemon, extracts the 8-char
topology prefix from every decnet_t_* resource, and force-removes
containers + networks whose prefix is not in the repo.
- POST /api/v1/topologies/reap-orphans (admin-only) returns a report
of live/orphan prefixes and what was removed.
- Resources belonging to live topologies are never touched; per-resource
errors are captured without aborting the sweep.
