Name AI assistant Hesychia in system prompts (#22)

## Summary
- Closes #20 — name the AI assistant **Hesychia** in system / generation prompts
- Add shared `assistant_identity.py` with `ASSISTANT_NAME` + concise calm/stillness tone
- Prepend identity to chat (`llm_service`), RAG, data analysis, and the views system message
- Document that identity lives in code (not env); add unit coverage

## Test plan
- [x] `uv run python manage.py test chat_backend.tests.test_assistant_identity chat_backend.tests.test_services_llm chat_backend.tests.test_services_data_analysis`
- [ ] Fresh chat: ask "who are you?" → responds as Hesychia
- [ ] Confirm classifiers/moderators/title generator unchanged (not assistant identity)

Related: companion frontend rebrand `chat_web_app#29`Reviewed-on: #22

README.md

@@ -95,6 +95,10 @@ with `COMPOSE_DATABASE_URL` if needed.
 | `GUNICORN_WORKERS` / `GUNICORN_BIND` | 2 / `0.0.0.0:8000` | optional | Entrypoint |
 | `SKIP_RAG_INIT` | unset | CI/migrate often `1` | Skip Chroma/Ollama boot work |
 
+Assistant identity (`Hesychia`) lives in code:
+`llm_be/chat_backend/services/assistant_identity.py` — prepended to user-facing
+generation prompts (chat, RAG, data analysis). Not env-configurable.
+
 Templates: `.env.example` (local), `.env.prod.example` (control-node secret).
 
 Control-node secret path (server-infra on ai-server-4080):