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/tasks/[id]/images/page.tsx

@@ -7,6 +7,7 @@ import { ImageProgressCardSkeleton } from '@/components/tasks/ImageProgressCard'
 import { ImageProgressHeader } from '@/components/tasks/ImageProgressHeader';
 import { ImageActionBar } from '@/components/tasks/ImageActionBar';
 import { SseStatusBar } from '@/components/tasks/SseStatusBar';
+import { FatalErrorBanner } from '@/components/tasks/FatalErrorBanner';
 import { useSse } from '@/hooks/useSse';
 import { useGenerationStore } from '@/stores/generationStore';
 import { useTaskStore } from '@/stores/taskStore';
@@ -50,6 +51,7 @@ export default function ImageSelectionPage() {
       )}
       <SseStatusBar status={connectionStatus} attempt={reconnectAttempt} />
       <div className="p-6 flex-1 overflow-auto">
+        <FatalErrorBanner />
         <ImageProgressHeader imageTotal={imageTotal} imageDone={imageDone} pendingCount={pendingCount} />
         {/* 图片网格（谁好谁先冒） */}
         <div className="grid grid-cols-3 xl:grid-cols-5 gap-3">