📦 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/cjs/isEmpty.js

@@ -0,0 +1,20 @@
+var _getLength = require('./_getLength.js');
+var isArray = require('./isArray.js');
+var isString = require('./isString.js');
+var isArguments = require('./isArguments.js');
+var keys = require('./keys.js');
+
+// Is a given array, string, or object empty?
+// An "empty" object has no enumerable own-properties.
+function isEmpty(obj) {
+  if (obj == null) return true;
+  // Skip the more expensive `toString`-based type checks if `obj` has no
+  // `.length`.
+  var length = _getLength(obj);
+  if (typeof length == 'number' && (
+    isArray(obj) || isString(obj) || isArguments(obj)
+  )) return length === 0;
+  return _getLength(keys(obj)) === 0;
+}
+
+module.exports = isEmpty;