📦 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/eslint-plugin-jest/docs/rules/prefer-to-have-length.md

@@ -0,0 +1,33 @@
+# Suggest using `toHaveLength()` (`prefer-to-have-length`)
+
+In order to have a better failure message, `toHaveLength()` should be used upon
+asserting expectations on objects length property.
+
+## Rule details
+
+This rule triggers a warning if `toBe()`, `toEqual()` or `toStrictEqual()` is
+used to assert objects length property.
+
+```js
+expect(files.length).toBe(1);
+```
+
+This rule is enabled by default.
+
+### Default configuration
+
+The following patterns are considered warnings:
+
+```js
+expect(files.length).toBe(1);
+
+expect(files.length).toEqual(1);
+
+expect(files.length).toStrictEqual(1);
+```
+
+The following pattern is not warning:
+
+```js
+expect(files).toHaveLength(1);
+```