feat(linkage): 计划-鞍座-实绩-停机联动 + 成本管理页

后端：
- 计划录入即「准备好」，队首(最早)自动「在线」(唯一)
- 新增上卷鞍座联动引擎 line_service：移动→鞍座→(有速度/投入生产)→生产中
  →带头达2000m→生产完成并自动产生实绩、持久化运行数据
- 停机自动检测：线速度为0持续>10min 自动新增待补充停机记录，恢复后自动结束
- /plan/start=移动到鞍座, 新增 /plan/{id}/commit 投入生产, /plan/saddle/current,
  /plan/seed 批量插入(轧制力模式)；后台引擎循环自动推进
- 新增成本管理：CostRecord 模型 + /cost CRUD + 9 类成本项(乳化液/盐酸/碱/电/水/蒸汽…)

前端：
- 入口跟踪重构为单个上卷鞍座工位(实时速度/带头长度进度/投入生产)+待上卷卡片+队列，
  计划列表/卡片/队列均可「移动」
- 新增成本管理页(成本项切换 + 柱+线图 + 明细表 + 时间筛选 + 新增)，布局参考乳化液耗量统计

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

frontend/src/api/index.js

@@ -20,9 +20,20 @@ export const getPlans = params => request.get('/plan/', { params })
 export const createPlan = data => request.post('/plan/', data)
 export const updatePlan = (id, data) => request.put(`/plan/${id}`, data)
 export const confirmPlan = id => request.patch(`/plan/${id}/confirm`)
-export const startProducing = id => request.patch(`/plan/${id}/start`)
+export const startProducing = id => request.patch(`/plan/${id}/start`)   // 移动到上卷鞍座
+export const moveToSaddle = id => request.patch(`/plan/${id}/start`)
+export const commitProducing = id => request.patch(`/plan/${id}/commit`) // 投入生产
+export const getSaddle = () => request.get('/plan/saddle/current')
+export const seedPlans = (count = 50) => request.post('/plan/seed', null, { params: { count } })
 export const getLastPlanTemplate = () => request.get('/plan/last-template')
 
+// 成本管理
+export const getCostItems = () => request.get('/cost/items')
+export const getCostRecords = params => request.get('/cost/', { params })
+export const createCostRecord = data => request.post('/cost/', data)
+export const updateCostRecord = (id, data) => request.put(`/cost/${id}`, data)
+export const deleteCostRecord = id => request.delete(`/cost/${id}`)
+
 // 停机管理
 export const getDowntimeCategories = () => request.get('/downtime/categories')
 export const getDowntimeRecords = params => request.get('/downtime/', { params })