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/_compat.py

@@ -0,0 +1,22 @@
+"""Platform compatibility."""
+
+import platform
+
+
+def get_startupinfo() -> None:
+    """Return None for startupinfo argument of ``subprocess.Popen``."""
+    return None
+
+
+assert get_startupinfo() is None, 'get_startupinfo() defaults to a no-op'
+
+
+if platform.system() == 'Windows':  # pragma: no cover
+    import subprocess
+
+    def get_startupinfo() -> subprocess.STARTUPINFO:  # pytype: disable=module-attr
+        """Return subprocess.STARTUPINFO instance hiding the console window."""
+        startupinfo = subprocess.STARTUPINFO()  # pytype: disable=module-attr
+        startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW  # pytype: disable=module-attr
+        startupinfo.wShowWindow = subprocess.SW_HIDE  # pytype: disable=module-attr
+        return startupinfo