fix: save button now fully persists canvas to DB

Root cause: handleAddNode/handleEdgeConfirm only updated Zustand store,
never creating records in the DB. canvasApi.save() only updated positions
of existing nodes, so nothing survived a page refresh.

Fix: save now sends the full canvas state (all nodes + edges + data) and
the backend does a full sync — upsert incoming, delete anything removed.
This means Save is the single source of truth: no need to call individual
create/update/delete APIs for every drag or edit.

backend/app/api/routes/canvas.py

@@ -23,12 +23,40 @@ async def load_canvas(db: AsyncSession = Depends(get_db), _: str = Depends(get_c
 
 @router.post("/save")
 async def save_canvas(body: CanvasSaveRequest, db: AsyncSession = Depends(get_db), _: str = Depends(get_current_user)):
-    # Update node positions from canvas
-    for node_pos in body.node_positions:
-        node = await db.get(Node, node_pos.id)
+    incoming_node_ids = {n.id for n in body.nodes}
+    incoming_edge_ids = {e.id for e in body.edges}
+
+    # Delete nodes removed from canvas
+    existing_nodes = (await db.execute(select(Node))).scalars().all()
+    for node in existing_nodes:
+        if node.id not in incoming_node_ids:
+            await db.delete(node)
+
+    # Delete edges removed from canvas
+    existing_edges = (await db.execute(select(Edge))).scalars().all()
+    for edge in existing_edges:
+        if edge.id not in incoming_edge_ids:
+            await db.delete(edge)
+
+    await db.flush()
+
+    # Upsert nodes
+    for node_data in body.nodes:
+        node = await db.get(Node, node_data.id)
         if node:
-            node.pos_x = node_pos.x
-            node.pos_y = node_pos.y
+            for field, value in node_data.model_dump().items():
+                setattr(node, field, value)
+        else:
+            db.add(Node(**node_data.model_dump()))
+
+    # Upsert edges
+    for edge_data in body.edges:
+        edge = await db.get(Edge, edge_data.id)
+        if edge:
+            for field, value in edge_data.model_dump().items():
+                setattr(edge, field, value)
+        else:
+            db.add(Edge(**edge_data.model_dump()))
 
     # Upsert viewport
     state = await db.get(CanvasState, 1)