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/encoding.py

@@ -0,0 +1,41 @@
+"""Encoding parameter handling and default."""
+
+import typing
+
+import codecs
+import locale
+
+from . import copying
+
+__all__ = ['DEFAULT_ENCODING', 'Encoding']
+
+DEFAULT_ENCODING = 'utf-8'
+
+
+class Encoding(copying.CopyBase):
+    """Encoding used for input and output with ``'utf-8'`` default."""
+
+    _encoding = DEFAULT_ENCODING
+
+    def __init__(self, *, encoding: typing.Optional[str] = DEFAULT_ENCODING,
+                 **kwargs) -> None:
+        super().__init__(**kwargs)
+
+        self.encoding = encoding
+
+    def _copy_kwargs(self, **kwargs):
+        """Return the kwargs to create a copy of the instance."""
+        return super()._copy_kwargs(encoding=self._encoding, **kwargs)
+
+    @property
+    def encoding(self) -> str:
+        """The encoding for the saved source file."""
+        return self._encoding
+
+    @encoding.setter
+    def encoding(self, encoding: typing.Optional[str]) -> None:
+        if encoding is None:
+            encoding = locale.getpreferredencoding()
+
+        codecs.lookup(encoding)  # raise early
+        self._encoding = encoding