NodeService.Delete dropped the node row (and its per-node child rows) without
checking for inbounds still referencing it via node_id, leaving orphaned
inbounds with a dangling node_id that confuse node sync, subscriptions and
cleanup. Refuse the delete with a clear error when inbounds are still attached,
and remove the per-node child rows before the node row inside one transaction.
Delete stays tolerant of a missing node row so it can still clean up orphaned
rows. Regression test covers the blocked and clean-delete paths.
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