📦 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/asynckit/stream.js

@@ -0,0 +1,21 @@
+var inherits              = require('util').inherits
+  , Readable              = require('stream').Readable
+  , ReadableAsyncKit      = require('./lib/readable_asynckit.js')
+  , ReadableParallel      = require('./lib/readable_parallel.js')
+  , ReadableSerial        = require('./lib/readable_serial.js')
+  , ReadableSerialOrdered = require('./lib/readable_serial_ordered.js')
+  ;
+
+// API
+module.exports =
+{
+  parallel      : ReadableParallel,
+  serial        : ReadableSerial,
+  serialOrdered : ReadableSerialOrdered, 
+};
+
+inherits(ReadableAsyncKit, Readable);
+
+inherits(ReadableParallel, ReadableAsyncKit);
+inherits(ReadableSerial, ReadableAsyncKit);
+inherits(ReadableSerialOrdered, ReadableAsyncKit);