The Show Port Numbers toggle reset on every reload because the backend
never stored it: the column was missing from the Node model and the field
was stripped by NodeSave/NodeBase/NodeUpdate schemas, so canvas/save
silently dropped it.
Add the show_port_numbers column (idempotent migration), and the field to
the node schemas so it round-trips through save/load.
Add regression tests covering persistence and the default-false case.
Fixes#184
ha-relevant: yes
Pouzor
2f3e986d4a