feat(linkage): 鞍座改为预备生产位 + 在线改人工触发 + 计划删除

- 上卷鞍座=预备生产位（不生产）；投入生产后离开鞍座、进入生产中并转入物料跟踪，鞍座随即空出
- 在线状态改为人工触发：移动到入口端才变在线，引擎不再自动上线(ensure_online 置空)
- 单卷在产：投入生产时若已有在产卷则拒绝
- 物料跟踪显示在产卷实时进度(客户端外推带头长度/进度条)
- 入口跟踪鞍座卡片改为预备生产展示(去掉进度)
- 计划管理新增删除按钮 + DELETE /plan/{id}(生产中不可删)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/plan.py

@@ -115,6 +115,18 @@ async def update_plan(
     return Response.ok(PlanOut.model_validate(plan))
 
 
+@router.delete("/{plan_id}", response_model=Response[dict])
+async def delete_plan(plan_id: int, db: AsyncSession = Depends(get_db), _ = Depends(get_current_user)):
+    result = await db.execute(select(ProductionPlan).where(ProductionPlan.id == plan_id))
+    plan = result.scalar_one_or_none()
+    if not plan:
+        raise HTTPException(status_code=404, detail="计划不存在")
+    if plan.status == "producing":
+        raise HTTPException(status_code=400, detail="生产中的计划不可删除")
+    await db.delete(plan)
+    return Response.ok({"deleted": plan_id})
+
+
 @router.patch("/{plan_id}/confirm", response_model=Response[PlanOut])
 async def confirm_plan(plan_id: int, db: AsyncSession = Depends(get_db), _ = Depends(get_current_user)):
     result = await db.execute(select(ProductionPlan).where(ProductionPlan.id == plan_id))
@@ -171,7 +183,10 @@ async def commit_producing(plan_id: int, db: AsyncSession = Depends(get_db), _ =
     plan = result.scalar_one_or_none()
     if not plan:
         raise HTTPException(status_code=404, detail="计划不存在")
-    await line_service.commit_plan(db, plan)
+    try:
+        await line_service.commit_plan(db, plan)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
     await db.flush()
     await db.refresh(plan)
     return Response.ok(PlanOut.model_validate(plan))