Thread-safe singleton for OpenAI client in Q&A agent

[Copilot]

Addresses review feedback: the OpenAI client was being instantiated on every function call, causing potential resource exhaustion under load.

Changes

• Add _get_openai_client() singleton helper with double-checked locking
• Replace direct OpenAI() instantiation in classify_with_instructor() and _generate_question_text()

_openai_client: OpenAI | None = None
_openai_client_lock = threading.Lock()

def _get_openai_client() -> OpenAI:
    """Return a thread-safe singleton OpenAI client instance."""
    global _openai_client
    if _openai_client is None:
        with _openai_client_lock:
            if _openai_client is None:
                _openai_client = OpenAI(api_key=settings.OPENAI_API_KEY)
    return _openai_client

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.