chore: mark `backend` folder as a Python project

deer-flow.code-workspace

@@ -1,14 +1,17 @@
 {
   "folders": [
-	{
-		"path": "."
-	},
-	{
-		"path": "backend"
-	},
-	{
-		"path": "frontend"
-	}
-],
-  "settings": {}
+    {
+      "path": "."
+    }
+  ],
+  "settings": {
+    "python-envs.pythonProjects": [
+      {
+        "path": "backend",
+        "envManager": "ms-python.python:venv",
+        "packageManager": "ms-python.python:pip",
+        "workspace": "deer-flow"
+      }
+    ]
+  }
 }