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/printing/gtk.py

@@ -0,0 +1,16 @@
+from sympy.printing.mathml import mathml
+from sympy.utilities.mathml import c2p
+import tempfile
+import subprocess
+
+
+def print_gtk(x, start_viewer=True):
+    """Print to Gtkmathview, a gtk widget capable of rendering MathML.
+
+    Needs libgtkmathview-bin"""
+    with tempfile.NamedTemporaryFile('w') as file:
+        file.write(c2p(mathml(x), simple=True))
+        file.flush()
+
+        if start_viewer:
+            subprocess.check_call(('mathmlviewer', file.name))