Add offline unit test suite for chat_backend (#12)

Closes #5

## Summary

- Replaces the three scattered test modules (`chat_backend/tests.py`, `services/tests.py`, `services/prompt_classifier/tests.py`) with a `chat_backend/tests/` package: **242 deterministic tests plus 6 opt-in live-Ollama checks**, up from 10 tests (3 of which were skipped and 4 of which were never even discovered).
- The suite runs fully offline — no Ollama, Chroma, SMTP or network access. LangChain runnables are replaced by a small `FakeChain`, Chroma/embeddings are mocked, email uses Django's locmem backend, and blobs go through `DatabaseStorage`.
- New `llm_be/test_runner.py` (wired via `TEST_RUNNER`) sets `SKIP_RAG_INIT=1` and an MD5 password hasher, so the suite cannot accidentally reach a model server and finishes in ~5s on SQLite (~17s on Postgres) instead of ~30s.

## Coverage

| Area | File |
|------|------|
| `TimeInfoBase.save`, slugs, `get_duration`, `file_exists`, cascades | `test_models.py` |
| `DatabaseStorage` save/open/exists/size/listdir/delete/times | `test_storage.py` |
| JWT claim, prompt/user/feedback/document serializers | `test_serializers.py` |
| auth + token, invite, feedback, company users, set-password, TOS | `test_views_users.py` |
| conversation list/order/create/detail/soft-delete | `test_views_conversations.py` |
| all four analytics endpoints, including empty-month behaviour | `test_views_analytics.py` |
| workspace + document upload/list/detail, 404 and 400 paths | `test_views_documents.py` |
| prompt classifier rules/parsing, moderation fail-safe, title cleanup | `test_services_classifiers.py` |
| CSV/XLSX/DOCX/PDF analysis, plot generation, error payloads | `test_services_data_analysis.py` |
| loader selection, filename sanitising, ingest, temp-file cleanup, search filters | `test_services_rag.py` |
| history formatting and streaming | `test_services_llm.py` |
| document re-index on create/delete, `SKIP_RAG_INIT` guard | `test_signals.py` |
| consumer DB helpers, LangGraph nodes (moderation, classification, generation, search flags), websocket routes | `test_consumers.py` |

Live checks (non-deterministic, need a model server):

```bash
cd llm_be
RUN_LIVE_OLLAMA_TESTS=1 uv run python manage.py test chat_backend.tests.test_live_ollama
```

## Bugs the tests surfaced (fixed here)

1. **`ConversationDetailView.post` silently dropped every prompt.** `import datetime` shadowed `from datetime import datetime`, so `datetime.now()` raised `AttributeError` inside a bare `except` and the endpoint returned 200 without saving. Now uses `timezone.now()`.
2. **Prompt attachments never reached the LLM.** `get_conversation_file_async` (both consumers) did `sync_to_async(prompt.file.read)` — with `DatabaseStorage` the attribute access itself opens the blob, i.e. a DB query in async context, raising `SynchronousOnlyOperation` that was swallowed and returned `(None, None)`. The read now happens inside the thread.
3. **`DatabaseStorage._save` crashed on a str-backed `ContentFile`** (`TypeError: sequence item 0: expected a bytes-like object`); chunks are encoded when needed.
4. **`services/prompt_classifier/__init__.,py`** (note the comma) meant the directory was only an implicit namespace package, which is why its test module was never collected. Renamed, and its duplicate live-Ollama tests folded into `test_live_ollama.py`.

Known-broken paths deliberately left untested and unchanged: `reset_password` / `ResetUserPassword` reference an unimported `requests` plus undefined locals, and `DocumentDetailView.get` references an undefined `workspaces` on its success path. Worth a follow-up ticket.

## Test plan

- [x] `cd llm_be && uv run python manage.py test` → 248 tests, OK (6 skipped, all opt-in live)
- [x] Same suite against Postgres 16 (`DATABASE_URL=postgres://…`) → OK, matching the containerized run in `deploy.yml`
- [x] `uv run black` clean on all added files
- [ ] Gitea Actions **Unit Tests** + **CI** green on this PRReviewed-on: #12

README.md

@@ -44,11 +44,24 @@ uv run python manage.py runserver 0.0.0.0:8003
 
 Without `DATABASE_URL` / `DB_HOST`, settings fall back to SQLite (`llm_be/db.sqlite3`).
 
-Tests (skip live-Ollama classifier cases):
+Tests:
 
 ```bash
 cd llm_be
-SKIP_RAG_INIT=1 uv run python manage.py test
+uv run python manage.py test
+```
+
+The suite is offline by default — the custom test runner
+(`llm_be/test_runner.py`) sets `SKIP_RAG_INIT=1` and a cheap password hasher, and
+LLM chains are faked, so no Ollama, Chroma, SMTP or network access is needed.
+Tests live in `llm_be/chat_backend/tests/` (models, storage, serializers, views,
+services, signals, consumers).
+
+Non-deterministic checks against a real model server are opt-in:
+
+```bash
+cd llm_be
+RUN_LIVE_OLLAMA_TESTS=1 uv run python manage.py test chat_backend.tests.test_live_ollama
 ```
 
 ### Docker (dev, bundled Postgres)