📦 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/lodash/xor.js

@@ -0,0 +1,28 @@
+var arrayFilter = require('./_arrayFilter'),
+    baseRest = require('./_baseRest'),
+    baseXor = require('./_baseXor'),
+    isArrayLikeObject = require('./isArrayLikeObject');
+
+/**
+ * Creates an array of unique values that is the
+ * [symmetric difference](https://en.wikipedia.org/wiki/Symmetric_difference)
+ * of the given arrays. The order of result values is determined by the order
+ * they occur in the arrays.
+ *
+ * @static
+ * @memberOf _
+ * @since 2.4.0
+ * @category Array
+ * @param {...Array} [arrays] The arrays to inspect.
+ * @returns {Array} Returns the new array of filtered values.
+ * @see _.difference, _.without
+ * @example
+ *
+ * _.xor([2, 1], [2, 3]);
+ * // => [1, 3]
+ */
+var xor = baseRest(function(arrays) {
+  return baseXor(arrayFilter(arrays, isArrayLikeObject));
+});
+
+module.exports = xor;