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/backend/unflattening.py

@@ -0,0 +1,63 @@
+"""Pipe DOT source code through ``unflatten``."""
+
+import pathlib
+import typing
+
+from ..encoding import DEFAULT_ENCODING
+from .. import _tools
+from .. import exceptions
+
+from . import execute
+
+__all__ = ['UNFLATTEN_BINARY', 'unflatten']
+
+UNFLATTEN_BINARY = pathlib.Path('unflatten')
+
+
+@_tools.deprecate_positional_args(supported_number=1)
+def unflatten(source: str,
+              stagger: typing.Optional[int] = None,
+              fanout: bool = False,
+              chain: typing.Optional[int] = None,
+              encoding: str = DEFAULT_ENCODING) -> str:
+    """Return DOT ``source`` piped through ``unflatten`` preprocessor as string.
+
+    Args:
+        source: DOT source to process
+            (improve layout aspect ratio).
+        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.
+        encoding: Encoding to encode unflatten stdin and decode its stdout.
+
+    Returns:
+        Decoded stdout of the Graphviz unflatten command.
+
+    Raises:
+        graphviz.RequiredArgumentError: If ``fanout`` is given
+            but no ``stagger``.
+        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
+    """
+    if fanout and stagger is None:
+        raise exceptions.RequiredArgumentError('fanout given without stagger')
+
+    cmd = [UNFLATTEN_BINARY]
+    if stagger is not None:
+        cmd += ['-l', str(stagger)]
+    if fanout:
+        cmd.append('-f')
+    if chain is not None:
+        cmd += ['-c', str(chain)]
+
+    proc = execute.run_check(cmd, input=source, encoding=encoding,
+                             capture_output=True)
+    return proc.stdout