📦 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/_replaceHolders.js

@@ -0,0 +1,29 @@
+/** Used as the internal argument placeholder. */
+var PLACEHOLDER = '__lodash_placeholder__';
+
+/**
+ * Replaces all `placeholder` elements in `array` with an internal placeholder
+ * and returns an array of their indexes.
+ *
+ * @private
+ * @param {Array} array The array to modify.
+ * @param {*} placeholder The placeholder to replace.
+ * @returns {Array} Returns the new array of placeholder indexes.
+ */
+function replaceHolders(array, placeholder) {
+  var index = -1,
+      length = array.length,
+      resIndex = 0,
+      result = [];
+
+  while (++index < length) {
+    var value = array[index];
+    if (value === placeholder || value === PLACEHOLDER) {
+      array[index] = PLACEHOLDER;
+      result[resIndex++] = index;
+    }
+  }
+  return result;
+}
+
+module.exports = replaceHolders;