Always-on grounded retrieval + role-scoped Ollama models (#62 Phases 1–3) (#65)

## Summary
- Closes Phases 1–3 of [#62](#62) (Phase 4 eval harness left for a follow-up).
- **Accuracy:** Retrieval is decided every turn (`GroundingDecider`, fails open). `FAST` no longer skips search — it only selects `OLLAMA_MODEL_FAST`. Search failures surface an explicit error instead of hallucinating from parametric memory.
- **Search:** Pluggable `services/search/` with **SearxNG primary** + DDGS failover, ranking/dedupe/rumour filtering, numbered dated source blocks, citations persisted on `Prompt.citations` and emitted as `{"v":1,"type":"citations",...}` after stream end.
- **Models:** Role-scoped `OLLAMA_MODEL_THINKING` / `_FAST` / `_UTILITY` / `OLLAMA_EMBED_MODEL=nomic-embed-text`, configurable `num_ctx`, real model name on `PromptMetric`, `reindex_embeddings` management command + loud embedding-dimension mismatch.

## SearxNG (ops)
See README **SearxNG** section. Short version: run `searxng/searxng` on the GPU host, enable `json` in `settings.yml`, set `SEARXNG_BASE_URL=http://10.0.0.128:8080` in prod/beta secrets, open `:8080` on the LAN firewall like Ollama.

## Test plan
- [x] `SKIP_RAG_INIT=1 python manage.py test chat_backend.tests` — 442 OK (6 skipped)
- [ ] Deploy beta with updated secrets (`OLLAMA_MODEL_*`, `OLLAMA_EMBED_MODEL=nomic-embed-text`, `SEARXNG_BASE_URL`)
- [ ] After embed change: `python manage.py reindex_embeddings`
- [ ] Verify `did Taylor Swift get married` in FAST and THINKING returns grounded answer with citations frame
- [ ] Kill SearxNG and confirm factual turns return search_unavailable (not Joe Alwyn hallucination); non-factual chat still worksReviewed-on: #65

pyproject.toml

@@ -42,6 +42,7 @@ dependencies = [
     "pillow==12.0.0",
     "beautifulsoup4==4.14.3",
     "ddgs==9.9.3",
+    "requests>=2.32,<3",
     "httpx==0.28.1",
     "python-dateutil==2.9.0.post0",
     "pytz==2025.2",