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/sympy/codegen/cxxnodes.py

@@ -0,0 +1,14 @@
+"""
+AST nodes specific to C++.
+"""
+
+from sympy.codegen.ast import Attribute, String, Token, Type, none
+
+class using(Token):
+    """ Represents a 'using' statement in C++ """
+    __slots__ = _fields = ('type', 'alias')
+    defaults = {'alias': none}
+    _construct_type = Type
+    _construct_alias = String
+
+constexpr = Attribute('constexpr')