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/solvers/benchmarks/bench_solvers.py

@@ -0,0 +1,12 @@
+from sympy.core.symbol import Symbol
+from sympy.matrices.dense import (eye, zeros)
+from sympy.solvers.solvers import solve_linear_system
+
+N = 8
+M = zeros(N, N + 1)
+M[:, :N] = eye(N)
+S = [Symbol('A%i' % i) for i in range(N)]
+
+
+def timeit_linsolve_trivial():
+    solve_linear_system(M, *S)