📦 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/readable-stream/readable-browser.js

@@ -0,0 +1,9 @@
+exports = module.exports = require('./lib/_stream_readable.js');
+exports.Stream = exports;
+exports.Readable = exports;
+exports.Writable = require('./lib/_stream_writable.js');
+exports.Duplex = require('./lib/_stream_duplex.js');
+exports.Transform = require('./lib/_stream_transform.js');
+exports.PassThrough = require('./lib/_stream_passthrough.js');
+exports.finished = require('./lib/internal/streams/end-of-stream.js');
+exports.pipeline = require('./lib/internal/streams/pipeline.js');