Fix ChatGroq invocation syntax (use .invoke instead of callable)

app.py

@@ -1034,7 +1034,7 @@ def generate_trivia():
         if not prompt:
             return jsonify({'success': False, 'error': 'No prompt provided'}), 400
         
-        response = chat_model([HumanMessage(content=prompt)])
+        response = chat_model.invoke([HumanMessage(content=prompt)])
         return jsonify({'success': True, 'response': response.content})
     except Exception as e:
         return jsonify({'success': False, 'error': str(e)}), 500