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/pybase64/_pybase64.pyi

@@ -0,0 +1,16 @@
+from ._typing import Buffer
+
+def _get_simd_flags_compile() -> int: ...
+def _get_simd_flags_runtime() -> int: ...
+def _get_simd_name(flags: int) -> str: ...
+def _get_simd_path() -> int: ...
+def _set_simd_path(flags: int) -> None: ...
+def b64decode(
+    s: str | Buffer, altchars: str | Buffer | None = None, validate: bool = False
+) -> bytes: ...
+def b64decode_as_bytearray(
+    s: str | Buffer, altchars: str | Buffer | None = None, validate: bool = False
+) -> bytearray: ...
+def b64encode(s: Buffer, altchars: str | Buffer | None = None) -> bytes: ...
+def b64encode_as_string(s: Buffer, altchars: str | Buffer | None = None) -> str: ...
+def encodebytes(s: Buffer) -> bytes: ...