📦 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/_createMathOperation.js

@@ -0,0 +1,38 @@
+var baseToNumber = require('./_baseToNumber'),
+    baseToString = require('./_baseToString');
+
+/**
+ * Creates a function that performs a mathematical operation on two values.
+ *
+ * @private
+ * @param {Function} operator The function to perform the operation.
+ * @param {number} [defaultValue] The value used for `undefined` arguments.
+ * @returns {Function} Returns the new mathematical operation function.
+ */
+function createMathOperation(operator, defaultValue) {
+  return function(value, other) {
+    var result;
+    if (value === undefined && other === undefined) {
+      return defaultValue;
+    }
+    if (value !== undefined) {
+      result = value;
+    }
+    if (other !== undefined) {
+      if (result === undefined) {
+        return other;
+      }
+      if (typeof value == 'string' || typeof other == 'string') {
+        value = baseToString(value);
+        other = baseToString(other);
+      } else {
+        value = baseToNumber(value);
+        other = baseToNumber(other);
+      }
+      result = operator(value, other);
+    }
+    return result;
+  };
+}
+
+module.exports = createMathOperation;