feat(linkage): 移动改为任意入口位置选择，仅上卷鞍座触发生产

- 计划新增 position 字段；新增 /plan/{id}/move?position=… 与 /plan/positions/all
- line_service.place_at_position：放到任意位置(位置唯一占用)，上卷鞍座单独触发生产联动
- 入口跟踪：新增入口位置图(单一鞍座)显示占位；移动按钮弹出位置选择框
- 计划管理：移动按钮同样弹出位置选择框

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

frontend/src/api/index.js

@@ -20,8 +20,10 @@ 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/${id}/start`)   // 移动到上卷鞍座
+export const startProducing = id => request.patch(`/plan/${id}/start`)   // 移动到上卷鞍座(兼容)
 export const moveToSaddle = id => request.patch(`/plan/${id}/start`)
+export const movePlan = (id, position) => request.patch(`/plan/${id}/move`, null, { params: { position } })
+export const getPositions = () => request.get('/plan/positions/all')
 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 } })