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>

services/document.py

@@ -1,11 +1,20 @@
+from pathlib import Path
+
 from api_client import APIClientProtocol
 
+_ALLOWED_EXTENSIONS = {".pdf", ".txt"}
+
 
 class DocumentService:
     def __init__(self, api_client: APIClientProtocol):
         self._api = api_client
 
     async def ingest(self, file_path: str) -> dict:
+        path = Path(file_path)
+        if not path.is_file():
+            raise ValueError(f"파일을 찾을 수 없습니다: {file_path}")
+        if path.suffix.lower() not in _ALLOWED_EXTENSIONS:
+            raise ValueError(f"지원하지 않는 파일 형식입니다: {path.suffix} (허용: pdf, txt)")
         return await self._api.ingest(file_path)
 
     async def list_documents(self) -> list[str]: