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

backend/app/constants/enums.py

@@ -23,6 +23,7 @@ class TaskStatus(str, Enum):
     APPROVED = "approved"
     REJECTED = "rejected"
     ARCHIVED = "archived"
+    FAILED = "failed"  # 生成彻底失败终态(重试耗尽):区分"没跑"和"跑挂了"(B6)
 
 
 # ── 审核状态（generation_tasks 平铺字段）──────────────────