📦 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/_Stack.js

@@ -0,0 +1,27 @@
+var ListCache = require('./_ListCache'),
+    stackClear = require('./_stackClear'),
+    stackDelete = require('./_stackDelete'),
+    stackGet = require('./_stackGet'),
+    stackHas = require('./_stackHas'),
+    stackSet = require('./_stackSet');
+
+/**
+ * Creates a stack cache object to store key-value pairs.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [entries] The key-value pairs to cache.
+ */
+function Stack(entries) {
+  var data = this.__data__ = new ListCache(entries);
+  this.size = data.size;
+}
+
+// Add methods to `Stack`.
+Stack.prototype.clear = stackClear;
+Stack.prototype['delete'] = stackDelete;
+Stack.prototype.get = stackGet;
+Stack.prototype.has = stackHas;
+Stack.prototype.set = stackSet;
+
+module.exports = Stack;