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/db/models.py

@@ -54,6 +54,7 @@ class Node(Base):
     ram_gb: Mapped[float | None] = mapped_column(Float, nullable=True)
     disk_gb: Mapped[float | None] = mapped_column(Float, nullable=True)
     show_hardware: Mapped[bool] = mapped_column(Boolean, default=False)
+    show_port_numbers: Mapped[bool] = mapped_column(Boolean, default=False)
     properties: Mapped[list[Any]] = mapped_column(JSON, default=list)
     width: Mapped[float | None] = mapped_column(Float, nullable=True)
     height: Mapped[float | None] = mapped_column(Float, nullable=True)