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/strategies/util.py

@@ -0,0 +1,17 @@
+from sympy.core.basic import Basic
+
+new = Basic.__new__
+
+
+def assoc(d, k, v):
+    d = d.copy()
+    d[k] = v
+    return d
+
+
+basic_fns = {'op': type,
+             'new': Basic.__new__,
+             'leaf': lambda x: not isinstance(x, Basic) or x.is_Atom,
+             'children': lambda x: x.args}
+
+expr_fns = assoc(basic_fns, 'new', lambda op, *args: op(*args))