Delete an empty private network (refuses while members exist)

Delete a private network, freeing its name and address block. SCOPE: one virtual network of your own workspace - the nodes themselves keep running, and create_network mints another any time. Refuses while ANY member rows exist: detach every node first (detach_network - detach is explicit, never a side effect of delete), and each detached address then sits in a 7-day quarantine that still counts as a member row - so a network becomes deletable at the earliest 7 days after its last detach (stale peers must drain before its addresses can ever mean someone else).