fix(agent): minimal safety net, zero-config router, LLM provider status UI

- Remove deterministic intent matching (keyword lists, infer_tool,
  fast_confirmation, negation guard, classification sets)
- Embedding descriptions auto-generated from tool docstrings
- LLM provider health endpoint GET /api/agent/llm-status
- 3 error codes: LLM_PROVIDER_UNAVAILABLE, LLM_TIMEOUT, LLM_AUTH_ERROR
- Frontend banner with auto-retry 30s + input disable
- i18n for LLM status messages (assistant.json ru/en)
- 138 passing backend tests

backend/requirements.txt

@@ -72,3 +72,4 @@ langchain-core>=0.3
 langchain-openai>=0.3
 langgraph-checkpoint-postgres
 pdfplumber
+sentence-transformers>=3.0