fix(api): remove duplicate to_dict() call in /api/graph/tasks

TaskManager.list_tasks() already returns a list of dicts (it calls
t.to_dict() internally at task.py:170). The endpoint then called
.to_dict() again on these plain dicts, raising AttributeError on
every request to GET /api/graph/tasks.

backend/app/api/graph.py

@@ -554,7 +554,7 @@ def list_tasks():
     
     return jsonify({
         "success": True,
-        "data": [t.to_dict() for t in tasks],
+        "data": tasks,
         "count": len(tasks)
     })