B6/B7: failed终态前端可见 + SSE事件链核实

B6 错误态:
- 后端已加 TaskStatus.FAILED + 异常区分重试中/耗尽(014迁移ALTER status ENUM)
- 前端 TaskStatus 类型加 failed
- RecentTasksBar/history STATUS_LABEL 补 failed 标签
- history 页 HISTORY_STATUSES 纳入 failed(否则失败任务前端找不到)
- FatalErrorBanner + useSse error 不再静默丢弃

B7: 核实 SSE(ticket换JWT) + 事件消费链无缺口,标绿

tsc EXIT=0

frontend/src/app/history/components.tsx

@@ -17,6 +17,7 @@ const STATUS_LABEL: Record<string, { text: string; color: string }> = {
   approved: { text: '已通过', color: 'text-green-600 bg-green-50' },
   archived: { text: '已归档', color: 'text-gray-500 bg-gray-100' },
   rejected: { text: '被打回', color: 'text-red-500 bg-red-50' },
+  failed: { text: '生成失败', color: 'text-red-600 bg-red-50' },
 };
 
 // --- DownloadButton --- 触发打包→轮询→下载 zip