chore: 添加虚拟环境到仓库

- 添加 backend_service/venv 虚拟环境
- 包含所有Python依赖包
- 注意：虚拟环境约393MB，包含12655个文件

backend_service/venv/lib/python3.13/site-packages/graphviz/parameters/base.py

@@ -0,0 +1,16 @@
+"""Rendering parameter handling."""
+
+from .. import copying
+
+__all__ = ['ParameterBase']
+
+
+class ParameterBase(copying.CopyBase):
+    """Rendering parameter."""
+
+    def _getattr_from_dict(self, attrname: str, *, default=None):
+        """Return self.attrname if attrname is in the instance dictionary
+            (as oposed to on the type)."""
+        if attrname in self.__dict__:
+            return getattr(self, attrname)
+        return default