feat(backend): 新增 /dashboard/rejected-tasks 驳回/返工下钻接口

- RejectedTask schema: 设备/工序/驳回人/返工负责人/原因/时间
- get_rejected_tasks 批量窗口取驳回人(reject log) + 复刻 reject_task 的返工负责人追溯逻辑(create log→父任务→自身)
- 复用 get_display_names 中文名映射、BEIJING_TZ 时间转换

backend/app/api/v1/endpoints/dashboard.py

@@ -9,6 +9,7 @@ from app.services.dashboard_service import (
     get_dashboard_stats, DashboardStats,
     get_wip_tasks, WipTask,
     get_completed_tasks, CompletedTask,
+    get_rejected_tasks, RejectedTask,
     get_people_workload, PersonWorkload,
     get_people_history, PersonHistoryRecord,
     search_product_messages, ProductMessageList,
@@ -56,6 +57,19 @@ async def completed_tasks(
     return await get_completed_tasks(db, since=since_dt, until=until_dt, limit=limit)
 
 
+@router.get("/rejected-tasks", response_model=list[RejectedTask])
+async def rejected_tasks(
+    since: str | None = Query(None, description="起始日期 ISO 如 2026-08-01T00:00:00"),
+    until: str | None = Query(None, description="截止日期 ISO"),
+    limit: int = Query(200, ge=1, le=500),
+    db: AsyncSession = Depends(get_db),
+):
+    """驳回/返工下钻 — 按时段查询被驳回任务明细（含返工去向）"""
+    since_dt = datetime.fromisoformat(since) if since else None
+    until_dt = datetime.fromisoformat(until) if until else None
+    return await get_rejected_tasks(db, since=since_dt, until=until_dt, limit=limit)
+
+
 @router.get("/people-workload", response_model=list[PersonWorkload])
 async def people_workload(
     db: AsyncSession = Depends(get_db),