Potential duplicate OWASP question classification in NestBot auto-reply flow

## Description

While reviewing the NestBot auto-reply flow, I noticed that OWASP question classification appears to run twice for messages that eventually receive an automated response.

Current flow:

1. `MessagePosted.handle_event()` calls `QuestionDetector.is_owasp_question()`
2. If classified as OWASP-related, the delayed auto-reply job is scheduled
3. `generate_ai_reply_if_unanswered()` later calls `process_ai_query()`
4. `process_ai_query()` creates another `QuestionDetector` instance and performs classification again

Relevant references:

* `backend/apps/slack/events/message_posted.py`
* `backend/apps/slack/services/message_auto_reply.py`
* `backend/apps/slack/common/handlers/ai.py`

Since `QuestionDetector` performs retrieval and an OpenAI-based classification step, this may result in duplicate work for messages that have already been validated before being queued.

## Questions

* Is the second classification intentional ?
* Would it make sense to reuse the original classification result for the auto-reply path ?
* Could this reduce OpenAI usage and retrieval overhead without changing behavior ?

Happy to investigate further if maintainers think this is worth pursuing.