fix: remove redundant .to_dict() call in list_tasks API

TaskManager.list_tasks() already returns serialized dicts, so calling
.to_dict() on each item causes AttributeError: 'dict' object has no
attribute 'to_dict'

Fixes: AttributeError in /api/graph/tasks endpoint

backend/app/api/graph.py

@@ -559,7 +559,7 @@ def list_tasks():
     
     return jsonify({
         "success": True,
-        "data": [t.to_dict() for t in tasks],
+        "data": tasks,  # list_tasks() already returns to_dict() results
         "count": len(tasks)
     })