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/utilities/magic.py

@@ -0,0 +1,12 @@
+"""Functions that involve magic. """
+
+def pollute(names, objects):
+    """Pollute the global namespace with symbols -> objects mapping. """
+    from inspect import currentframe
+    frame = currentframe().f_back.f_back
+
+    try:
+        for name, obj in zip(names, objects):
+            frame.f_globals[name] = obj
+    finally:
+        del frame  # break cyclic dependencies as stated in inspect docs