📦 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/Issue.d.ts

@@ -0,0 +1,13 @@
+import { IssueSeverity } from './IssueSeverity';
+import { IssueLocation } from './IssueLocation';
+interface Issue {
+    origin: string;
+    severity: IssueSeverity;
+    code: string;
+    message: string;
+    file?: string;
+    location?: IssueLocation;
+}
+declare function isIssue(value: unknown): value is Issue;
+declare function deduplicateAndSortIssues(issues: Issue[]): Issue[];
+export { Issue, isIssue, deduplicateAndSortIssues };