feat(classifier): optional GigaChat-2-Lite hybrid tier classify

LLM classify via LiteLLM gigachat-classifier when heuristic confidence
is low. CLASSIFIER_MODE=heuristic|hybrid|llm. Metrics classifier_source.

README.md

@@ -88,6 +88,26 @@ Novita always direct (`NO_PROXY=api.novita.ai`).
 
 Header `X-AI-Quality: auto|economy|balanced|max` or `metadata.quality_mode` in request body. Default: `auto` (start lane A/B by tier, escalate on retry/5xx).
 
+## LLM classifier (GigaChat, optional)
+
+When `CLASSIFIER_MODE=hybrid` (default) and `GIGACHAT_CREDENTIALS` is set:
+
+1. **Heuristic** classify first (0 cost)
+2. If `confidence < 0.6` → one call to **GigaChat-2-Lite** via LiteLLM (`gigachat-classifier`)
+3. Lane orchestration unchanged (A/B/C, Redis, budget)
+
+| `CLASSIFIER_MODE` | Behavior |
+|-------------------|----------|
+| `heuristic` | Keywords only (no GigaChat) |
+| `hybrid` | GigaChat only on low confidence |
+| `llm` | Always GigaChat for text (except vision) |
+
+Freemium GigaChat — для личного некомmercial теста; prod — юр. тариф Сбера.
+
+Setup: [developers.sber.ru](https://developers.sber.ru/docs/ru/gigachat/quickstart/ind-create-project) → Authorization key → `.env` `GIGACHAT_CREDENTIALS`.
+
+Response field: `x_router_meta.classifier_source` = `heuristic` | `gigachat` | `heuristic_fallback`.
+
 ## Response metadata
 
 Each chat response includes `x_router_meta`: