feat: [FL-184] 添加调试模式配置以返回异常堆栈跟踪

- 在配置文件中添加 debug_mode 参数，默认值为 true
- 创建全局异常处理器，当 debug_mode 开启时返回 stacktrace 信息
- 在 .env.example 中添加 DEBUG_MODE 配置项说明
- 新增测试文件验证调试模式功能

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

api/tests/test_debug_mode.py

@@ -0,0 +1,59 @@
+"""Test debug mode configuration and exception handling."""
+from unittest.mock import patch
+
+from fastapi import Request
+from fastapi.responses import JSONResponse
+
+from app.core.config import Settings, get_settings
+from app.core.exception_handlers import global_exception_handler
+
+
+def test_debug_mode_default_enabled():
+    """Test that debug_mode defaults to True."""
+    settings = Settings()
+    assert settings.debug_mode is True
+
+
+def test_debug_mode_can_be_disabled():
+    """Test that debug_mode can be disabled via environment."""
+    with patch.dict("os.environ", {"DEBUG_MODE": "false"}):
+        settings = Settings()
+        assert settings.debug_mode is False
+
+
+async def test_exception_handler_with_debug_enabled():
+    """Test exception handler includes stacktrace when debug is enabled."""
+    with patch("app.core.exception_handlers.get_settings") as mock_settings:
+        mock_settings.return_value.debug_mode = True
+
+        request = None
+        exc = ValueError("Test error")
+
+        response = await global_exception_handler(request, exc)
+
+        assert isinstance(response, JSONResponse)
+        assert response.status_code == 500
+
+        content = response.body.decode()
+        assert "Test error" in content
+        assert "ValueError" in content
+        assert "stacktrace" in content
+
+
+async def test_exception_handler_with_debug_disabled():
+    """Test exception handler excludes stacktrace when debug is disabled."""
+    with patch("app.core.exception_handlers.get_settings") as mock_settings:
+        mock_settings.return_value.debug_mode = False
+
+        request = None
+        exc = ValueError("Test error")
+
+        response = await global_exception_handler(request, exc)
+
+        assert isinstance(response, JSONResponse)
+        assert response.status_code == 500
+
+        content = response.body.decode()
+        assert "Test error" in content
+        assert "ValueError" in content
+        assert "stacktrace" not in content