📦 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/experimentalWarning.js

@@ -0,0 +1,17 @@
+'use strict'
+
+var experimentalWarnings = new Set();
+
+function emitExperimentalWarning(feature) {
+  if (experimentalWarnings.has(feature)) return;
+  var msg = feature + ' is an experimental feature. This feature could ' +
+       'change at any time';
+  experimentalWarnings.add(feature);
+  process.emitWarning(msg, 'ExperimentalWarning');
+}
+
+function noop() {}
+
+module.exports.emitExperimentalWarning = process.emitWarning
+  ? emitExperimentalWarning
+  : noop;