delete_topology_cascade manually deletes status_events, edges, deckies
and lans but overlooked topology_mutations, so deleting any topology
that ever had a mutation enqueued (i.e. edits while active|degraded)
failed with an FK IntegrityError. Add the missing DELETE and extend
the cascade test to seed a mutation row.
Adds topology CRUD to BaseRepository (NotImplementedError defaults) and
implements them in SQLModelRepository: create/get/list/delete topologies,
add/update/list LANs and TopologyDeckies, add/list edges, plus an atomic
update_topology_status that appends a TopologyStatusEvent in the same
transaction. Cascade delete sweeps children before the topology row.
Covered by tests/topology/test_repo.py (roundtrip, per-topology name
uniqueness, status event log, cascade delete, status filter) and an
extension to tests/test_base_repo.py for the NotImplementedError surface.
