📦 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/fork-ts-checker-webpack-plugin/lib/issue/IssueSeverity.js

@@ -0,0 +1,11 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+function isIssueSeverity(value) {
+    return ['error', 'warning'].includes(value);
+}
+exports.isIssueSeverity = isIssueSeverity;
+function compareIssueSeverities(severityA, severityB) {
+    const [priorityA, priorityB] = [severityA, severityB].map((severity) => ['warning' /* 0 */, 'error' /* 1 */].indexOf(severity));
+    return Math.sign(priorityB - priorityA);
+}
+exports.compareIssueSeverities = compareIssueSeverities;