Phase 27: P2 quality improvements — logging, httpx pooling, validation, tests

- app.py: replace print() with logging, basicConfig with LOG_LEVEL env var
- api_client.py: shared AsyncClient instance (connection pooling), URL-encode
  delete_document path parameter, aclose() for cleanup
- services/document.py: validate file exists and extension before ingest
- tests/: ChatService (4) + DocumentService (6) unit tests via pytest-asyncio
- pyproject.toml: asyncio_mode = auto
- requirements-dev.txt: pytest, pytest-asyncio

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -8,6 +8,7 @@
     YOULBOT_API_TOKEN=           ← api.py에 API_TOKEN 설정 시 동일 값
 """
 import html as _html
+import logging
 import os
 
 import gradio as gr
@@ -15,6 +16,13 @@ from dotenv import load_dotenv
 
 load_dotenv()
 
+logging.basicConfig(
+    level=os.getenv("LOG_LEVEL", "INFO").upper(),
+    format="%(asctime)s %(levelname)-8s %(name)s — %(message)s",
+    datefmt="%Y-%m-%d %H:%M:%S",
+)
+logger = logging.getLogger(__name__)
+
 from config import AppConfig
 from container import Container
 
@@ -136,7 +144,7 @@ async def handle_feedback(like_data: gr.LikeData, history, run_ids, user_id):
     try:
         await container.chat_service.save_feedback(user_id, user_msg, asst_msg, rating, run_id)
     except Exception as e:
-        print(f"[Feedback] 저장 실패: {e}")
+        logger.error("피드백 저장 실패: %s", e)
 
 
 def switch_user(user_id):
@@ -147,7 +155,7 @@ async def reset_chat(user_id):
     try:
         await container.chat_service.reset(user_id)
     except Exception as e:
-        print(f"[Reset] 실패: {e}")
+        logger.error("대화 초기화 실패: %s", e)
     return [], []