📦 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/underscore/modules/difference.js

@@ -0,0 +1,13 @@
+import restArguments from './restArguments.js';
+import flatten from './_flatten.js';
+import filter from './filter.js';
+import contains from './contains.js';
+
+// Take the difference between one array and a number of other arrays.
+// Only the elements present in just the first array will remain.
+export default restArguments(function(array, rest) {
+  rest = flatten(rest, true, true);
+  return filter(array, function(value){
+    return !contains(rest, value);
+  });
+});