change back to gpt 5 mini.

app.py

@@ -26,7 +26,7 @@ def call_model(system_prompt: str, history: list[dict[str, str]]) -> str:
     typed_msgs = cast(List[ChatCompletionMessageParam], msgs)
     
     resp = client.chat.completions.create(
-        model="gpt-4o",
+        model="gpt-5-mini",
         messages=typed_msgs,
         temperature=0.7,
     )