fix: persist top/left/right connection-point counts (issue #243)

The backend only stored bottom_handles, so top/left/right_handles were dropped
on canvas save and reset to defaults on reload — a left/right snappoint would
vanish after reload.

- models.py: add top_handles (default 1), left_handles (0), right_handles (0)
- database.py: ALTER TABLE migrations for the three columns
- schemas: add fields to NodeBase, NodeUpdate, and the canvas-save node schema
- tests: save+reload round-trip and default-fallback coverage

ha-relevant: no

backend/app/db/models.py

@@ -59,6 +59,9 @@ class Node(Base):
     width: Mapped[float | None] = mapped_column(Float, nullable=True)
     height: Mapped[float | None] = mapped_column(Float, nullable=True)
     bottom_handles: Mapped[int] = mapped_column(Integer, default=1)
+    top_handles: Mapped[int] = mapped_column(Integer, default=1)
+    left_handles: Mapped[int] = mapped_column(Integer, default=0)
+    right_handles: Mapped[int] = mapped_column(Integer, default=0)
     ieee_address: Mapped[str | None] = mapped_column(String, index=True, nullable=True)
     last_seen: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     last_scan: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))