📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/postcss-font-variant/CHANGELOG.md

@@ -0,0 +1,48 @@
+# 5.0.0 - 2021-01-12
+
+- Changed: use PostCSS 7 API
+- Updated: Node support to 10.0.0 (major)
+
+# 4.0.1 - 2020-10-28
+
+- Fixed: Incorrect conversion of small-caps ([#15](https://github.com/postcss/postcss-font-variant/pull/15))
+
+# 4.0.0 - 2018-09-17
+
+- Changed: use PostCSS 7 API
+
+# 3.0.0 - 2017-05-11
+
+- Changed: use PostCSS 6 API
+
+# 2.0.1 - 2016-07-08
+
+- Fixed: existing font-feature-settings being duplicated.
+  ([#8](https://github.com/postcss/postcss-font-variant/pull/8) - @ChaosExAnima)
+
+# 2.0.0 - 2015-09-08
+
+- Added: compatibility with postcss v5.x
+- Removed: compatiblity with postcss v4.x
+
+# 1.2.0 - 2015-08-13
+
+- Added: compatibility with postcss v4.1.x
+  ([#5](https://github.com/postcss/postcss-font-variant/pull/5))
+
+# 1.1.0 - 2015-01-29
+
+  - Fixed: Properly handle font-variant-position:normal ([#3](https://github.com/postcss/postcss-font-variant/pull/3))
+  - Added: support font-kerning ([#2](https://github.com/postcss/postcss-font-variant/pull/2))
+
+# 1.0.2 - 2015-01-27
+
+- Fixed: Re–use existing font-feature-settings declarations to avoid creating multiples that override themselves ([#1](https://github.com/postcss/postcss-font-variant/pull/1))
+
+# 1.0.1 - 2014-11-11
+
+- Fixed: wrong space char that breaks on some environnements
+
+# 1.0.0 - 2014-10-09
+
+✨ First release based on [rework-font-variant](https://github.com/ianstormtaylor/rework-font-variant) v1.0.1