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/page.tsx

@@ -11,9 +11,9 @@ import { getErrorAction } from '@/types/errors';
 import { clsx } from 'clsx';
 import { HistoryTable, HistorySkeleton, Pagination } from './components';
 
-const HISTORY_STATUSES = ['approved', 'archived', 'rejected'];
+const HISTORY_STATUSES = ['approved', 'archived', 'rejected', 'failed'];
 const STATUS_LABELS: Record<string, string> = {
-  approved: '已通过', archived: '已归档', rejected: '被打回',
+  approved: '已通过', archived: '已归档', rejected: '被打回', failed: '生成失败',
 };
 const PAGE_SIZE = 20;