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/cutils.py

@@ -0,0 +1,8 @@
+from sympy.printing.c import C99CodePrinter
+
+def render_as_source_file(content, Printer=C99CodePrinter, settings=None):
+    """ Renders a C source file (with required #include statements) """
+    printer = Printer(settings or {})
+    code_str = printer.doprint(content)
+    includes = '\n'.join(['#include <%s>' % h for h in printer.headers])
+    return includes + '\n\n' + code_str