📦 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/memfs/lib/internal/buffer.d.ts

@@ -0,0 +1,15 @@
+/// <reference types="node" />
+import { Buffer } from 'buffer';
+declare const bufferAllocUnsafe: (size: number) => Buffer;
+declare const bufferFrom: {
+    (arrayBuffer: ArrayBuffer | SharedArrayBuffer, byteOffset?: number | undefined, length?: number | undefined): Buffer;
+    (data: readonly any[]): Buffer;
+    (data: Uint8Array): Buffer;
+    (obj: {
+        valueOf(): string | object;
+    } | {
+        [Symbol.toPrimitive](hint: "string"): string;
+    }, byteOffset?: number | undefined, length?: number | undefined): Buffer;
+    (str: string, encoding?: string | undefined): Buffer;
+};
+export { Buffer, bufferAllocUnsafe, bufferFrom };