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/unflattening.py

@@ -0,0 +1,63 @@
+"""Pipe source through the Graphviz *unflatten* preprocessor."""
+
+import typing
+
+import graphviz
+from . import _tools
+from . import base
+from . import backend
+from . import encoding
+
+__all__ = ['Unflatten']
+
+
+class Unflatten(encoding.Encoding, base.Base, backend.Unflatten):
+    """Pipe source through the Graphviz *unflatten* preprocessor."""
+
+    @_tools.deprecate_positional_args(supported_number=0, ignore_arg='self')
+    def unflatten(self,
+                  stagger: typing.Optional[int] = None,
+                  fanout: bool = False,
+                  chain: typing.Optional[int] = None) -> 'graphviz.Source':
+        """Return a new :class:`.Source` instance with the source
+            piped through the Graphviz *unflatten* preprocessor.
+
+        Args:
+            stagger: Stagger the minimum length
+                of leaf edges between 1 and this small integer.
+            fanout: Fanout nodes with indegree = outdegree = 1
+                when staggering (requires ``stagger``).
+            chain: Form disconnected nodes into chains
+                of up to this many nodes.
+
+        Returns:
+            Prepocessed DOT source code (improved layout aspect ratio).
+
+        Raises:
+            graphviz.RequiredArgumentError: If ``fanout`` is given
+                but ``stagger`` is None.
+            graphviz.ExecutableNotFound: If the Graphviz ``unflatten`` executable
+                is not found.
+            graphviz.CalledProcessError: If the returncode (exit status)
+                of the unflattening 'unflatten' subprocess is non-zero.
+
+        See also:
+            Upstream documentation:
+            https://www.graphviz.org/pdf/unflatten.1.pdf
+        """
+        from . import sources
+
+        out = self._unflatten(self.source,
+                              stagger=stagger, fanout=fanout, chain=chain,
+                              encoding=self.encoding)
+
+        kwargs = self._copy_kwargs()
+        return sources.Source(out,
+                              filename=kwargs.get('filename'),
+                              directory=kwargs.get('directory'),
+                              format=kwargs.get('format'),
+                              engine=kwargs.get('engine'),
+                              encoding=kwargs.get('encoding'),
+                              renderer=kwargs.get('renderer'),
+                              formatter=kwargs.get('formatter'),
+                              loaded_from_path=None)