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/base.py

@@ -0,0 +1,32 @@
+"""Iterables of DOT source code lines (including final newline)."""
+
+import typing
+
+from . import copying
+
+__all__ = ['Base']
+
+
+class LineIterable:
+    """Iterable of DOT Source code lines
+        (mimics ``file`` objects in text mode)."""
+
+    def __iter__(self) -> typing.Iterator[str]:  # pragma: no cover
+        r"""Yield the generated DOT source line by line.
+
+        Yields: Line ending with a newline (``'\n'``).
+        """
+        raise NotImplementedError('to be implemented by concrete subclasses')
+
+
+# Common base interface for all exposed classes
+class Base(LineIterable, copying.CopyBase):
+    """LineIterator with ``.source`` attribute, that it returns for ``str()``."""
+
+    @property
+    def source(self) -> str:  # pragma: no cover
+        raise NotImplementedError('to be implemented by concrete subclasses')
+
+    def __str__(self) -> str:
+        """The DOT source code as string."""
+        return self.source