📦 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/difference.js

@@ -0,0 +1,33 @@
+var baseDifference = require('./_baseDifference'),
+    baseFlatten = require('./_baseFlatten'),
+    baseRest = require('./_baseRest'),
+    isArrayLikeObject = require('./isArrayLikeObject');
+
+/**
+ * Creates an array of `array` values not included in the other given arrays
+ * using [`SameValueZero`](http://ecma-international.org/ecma-262/7.0/#sec-samevaluezero)
+ * for equality comparisons. The order and references of result values are
+ * determined by the first array.
+ *
+ * **Note:** Unlike `_.pullAll`, this method returns a new array.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Array
+ * @param {Array} array The array to inspect.
+ * @param {...Array} [values] The values to exclude.
+ * @returns {Array} Returns the new array of filtered values.
+ * @see _.without, _.xor
+ * @example
+ *
+ * _.difference([2, 1], [2, 3]);
+ * // => [1]
+ */
+var difference = baseRest(function(array, values) {
+  return isArrayLikeObject(array)
+    ? baseDifference(array, baseFlatten(values, 1, isArrayLikeObject, true))
+    : [];
+});
+
+module.exports = difference;