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/this.py

@@ -0,0 +1,21 @@
+"""
+The Zen of SymPy.
+"""
+
+s = """The Zen of SymPy
+
+Unevaluated is better than evaluated.
+The user interface matters.
+Printing matters.
+Pure Python can be fast enough.
+If it's too slow, it's (probably) your fault.
+Documentation matters.
+Correctness is more important than speed.
+Push it in now and improve upon it later.
+Coverage by testing matters.
+Smart tests are better than random tests.
+But random tests sometimes find what your smartest test missed.
+The Python way is probably the right way.
+Community is more important than code."""
+
+print(s)