change free open router ai model

app.py

@@ -53,7 +53,7 @@ def explain(req: ExplainRequest):
         "Content-Type": "application/json"
     }
     payload = {
-        "model": "x-ai/grok-4-fast:free",
+        "model": "nvidia/nemotron-nano-12b-v2-vl:free",
         "messages": [
             {"role": "system", "content": system_prompt},
             {"role": "user", "content": user_message}