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/alembic/versions/014_task_status_failed.py

@@ -0,0 +1,34 @@
+"""014 generation_tasks.status ENUM 补 failed 终态（B6 错误态）
+
+Revision ID: 014
+Revises: 013
+Create Date: 2026-06-16
+
+status 是 MySQL ENUM,原7值无失败终态。Celery生成彻底失败(重试耗尽)时
+要落 failed,区分"没跑(pending)"和"跑挂了(failed)"。不补 ENUM 会 Data truncated。
+"""
+from alembic import op
+
+revision = "014"
+down_revision = "013"
+branch_labels = None
+depends_on = None
+
+_ENUM_WITH_FAILED = (
+    "ENUM('pending','generating','pending_selection','pending_review',"
+    "'approved','rejected','archived','failed') NOT NULL DEFAULT 'pending'"
+)
+_ENUM_ORIG = (
+    "ENUM('pending','generating','pending_selection','pending_review',"
+    "'approved','rejected','archived') NOT NULL DEFAULT 'pending'"
+)
+
+
+def upgrade():
+    op.execute(f"ALTER TABLE generation_tasks MODIFY COLUMN status {_ENUM_WITH_FAILED}")
+
+
+def downgrade():
+    # 回退前先把 failed 行归位到 pending,避免越界
+    op.execute("UPDATE generation_tasks SET status='pending' WHERE status='failed'")
+    op.execute(f"ALTER TABLE generation_tasks MODIFY COLUMN status {_ENUM_ORIG}")