fix: persist edge waypoints in backend

Add waypoints JSON column to edges table, include it in all edge
schemas (EdgeBase, EdgeUpdate, canvas CanvasEdge) and add the
idempotent ALTER TABLE migration so existing databases are upgraded
on next startup.

backend/app/db/models.py

@@ -69,6 +69,7 @@ class Edge(Base):
     animated: Mapped[str] = mapped_column(String, nullable=False, default='none')
     source_handle: Mapped[str | None] = mapped_column(String)
     target_handle: Mapped[str | None] = mapped_column(String)
+    waypoints: Mapped[list | None] = mapped_column(JSON, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)