fix: add missing logger import in rag_chat_use_case

- Fixes NameError: name 'logger' is not defined
- Resolves 500 Internal Server Error on /api/v1/rag/chat/stream

src/core/use_cases/rag_chat_use_case.py

@@ -1,6 +1,7 @@
 import tiktoken
 import asyncio
 import concurrent.futures
+import logging
 from datetime import datetime
 from typing import List, Dict, Any, Tuple, AsyncGenerator, Optional
 
@@ -17,6 +18,8 @@ from src.core.domain.schemas import ChatRequest # We assume this still exists
 from src.core.orchestrator.query_orchestrator import QueryOrchestrator
 from src.core.ranking.hybrid_result_ranker import HybridResultRanker
 
+logger = logging.getLogger(__name__)
+
 # Supported languages for multilingual parallel search
 # Maps language code → human name (used in prompts and source labels)
 SUPPORTED_LANGUAGES = {