📦 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/no-jest-import.md

@@ -0,0 +1,20 @@
+# Disallow importing Jest (`no-jest-import`)
+
+The `jest` object is automatically in scope within every test file. The methods
+in the `jest` object help create mocks and let you control Jest's overall
+behavior. It is therefore completely unnecessary to import in `jest`, as Jest
+doesn't export anything in the first place.
+
+### Rule details
+
+This rule reports on any importing of Jest.
+
+To name a few: `var jest = require('jest');` `const jest = require('jest');`
+`import jest from 'jest';` `import {jest as test} from 'jest';`
+
+There is no correct usage of this code, other than to not import `jest` in the
+first place.
+
+## Further Reading
+
+\*[The Jest Object](https://facebook.github.io/jest/docs/en/jest-object.html)