fix(nodes): persist show_port_numbers across reloads

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

backend/app/schemas/canvas.py

@@ -29,6 +29,7 @@ class NodeSave(BaseModel):
     ram_gb: float | None = None
     disk_gb: float | None = None
     show_hardware: bool = False
+    show_port_numbers: bool = False
     properties: list[Any] = []
     width: float | None = None
     height: float | None = None