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>

tests/test_chat_service.py

@@ -0,0 +1,40 @@
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+
+from services.chat import ChatService
+
+
+@pytest.fixture
+def mock_api():
+    api = MagicMock()
+    api.reset = AsyncMock()
+    api.save_feedback = AsyncMock()
+    return api
+
+
+@pytest.fixture
+def service(mock_api):
+    return ChatService(mock_api)
+
+
+async def test_reset_delegates_to_api(service, mock_api):
+    await service.reset("user1")
+    mock_api.reset.assert_awaited_once_with("user1")
+
+
+async def test_save_feedback_delegates_to_api(service, mock_api):
+    await service.save_feedback("user1", "질문", "답변", 1, "run-123")
+    mock_api.save_feedback.assert_awaited_once_with("user1", "질문", "답변", 1, "run-123")
+
+
+async def test_save_feedback_with_no_run_id(service, mock_api):
+    await service.save_feedback("user1", "질문", "답변", -1, None)
+    mock_api.save_feedback.assert_awaited_once_with("user1", "질문", "답변", -1, None)
+
+
+def test_chat_returns_api_iterator(service, mock_api):
+    sentinel = object()
+    mock_api.chat = MagicMock(return_value=sentinel)
+    result = service.chat("안녕", "user1", False)
+    assert result is sentinel
+    mock_api.chat.assert_called_once_with("안녕", "user1", False)