📦 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/@pmmmwh/react-refresh-webpack-plugin/loader/utils/getIdentitySourceMap.js

@@ -0,0 +1,30 @@
+const { SourceMapGenerator } = require('source-map');
+
+/**
+ * Generates an identity source map from a source file.
+ * @param {string} source The content of the source file.
+ * @param {string} resourcePath The name of the source file.
+ * @returns {import('source-map').RawSourceMap} The identity source map.
+ */
+function getIdentitySourceMap(source, resourcePath) {
+  const sourceMap = new SourceMapGenerator();
+  sourceMap.setSourceContent(resourcePath, source);
+
+  source.split('\n').forEach((line, index) => {
+    sourceMap.addMapping({
+      source: resourcePath,
+      original: {
+        line: index + 1,
+        column: 0,
+      },
+      generated: {
+        line: index + 1,
+        column: 0,
+      },
+    });
+  });
+
+  return sourceMap.toJSON();
+}
+
+module.exports = getIdentitySourceMap;