## Summary
- Closes [#33](#33) — per-user `use_conversation_context` boolean on `CustomUser` (default `false`, opt-in).
- Migration `0034_customuser_use_conversation_context`.
- `GET/POST /api/conversation_preferences` reads/writes the flag; POST sets absolute boolean without toggling `conversation_order` when only this field is sent.
- Chat + document RAG paths skip prior-turn history when the flag is off (`AsyncLLMService`, `AsyncRAGService`, both WS consumers).
## Frontend contract
- Field: `use_conversation_context` (boolean, default false)
- Read: `GET /api/conversation_preferences` → `{ order, use_conversation_context }`
- Write: `POST /api/conversation_preferences` with `{ "use_conversation_context": true|false }`
- Tooltip copy: use previous conversations to better customize the experience
- Counterpart: [chat_web_app#66](ai_ml_operations/chat_web_app#66)
## Test plan
- [ ] `manage.py test chat_backend.tests.test_models chat_backend.tests.test_views_conversations.ConversationPreferencesTestCase chat_backend.tests.test_services_llm chat_backend.tests.test_services_rag`
- [ ] New user / unset flag → `use_conversation_context` is false; history empty in LLM/RAG prompts
- [ ] Enable via preferences API → prior turns appear in history
- [ ] Disable again → history skipped; order preference unchanged when posting only the context flagReviewed-on: #73
westfarn