feat(backend): 新增 /dashboard/wip-matrix 在制品交叉聚合接口

- 规格型号×人员/工序 的设备数量聚合(WIP/PENDING任务)
- dimension 参数: assignee(中文姓名)/task_name(工序名)
- 返回扁平数组含 count + assignees(该交叉点主负责人列表)

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

@@ -12,6 +12,7 @@ from app.services.dashboard_service import (
     get_rejected_tasks, RejectedTask,
     get_user_operations, UserOperation,
     get_user_operation_detail, OperationDetail,
+    get_wip_matrix, WipMatrixRow,
     get_people_workload, PersonWorkload,
     get_people_history, PersonHistoryRecord,
     search_product_messages, ProductMessageList,
@@ -100,6 +101,15 @@ async def user_operations_detail(
     )
 
 
+@router.get("/wip-matrix", response_model=list[WipMatrixRow])
+async def wip_matrix(
+    dimension: str = Query("assignee", description="聚合维度: assignee(人员) / task_name(工序)"),
+    db: AsyncSession = Depends(get_db),
+):
+    """在制品分布透视表 — 规格型号 × 人员/工序 的设备数量交叉聚合"""
+    return await get_wip_matrix(db, dimension=dimension)
+
+
 @router.get("/people-workload", response_model=list[PersonWorkload])
 async def people_workload(
     db: AsyncSession = Depends(get_db),