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>

backend/app/api/plan.py

@@ -142,6 +142,28 @@ async def move_to_saddle(plan_id: int, db: AsyncSession = Depends(get_db), _ = D
     return Response.ok(PlanOut.model_validate(plan))
 
 
+@router.get("/positions/all", response_model=Response[list[str]])
+async def list_positions(_ = Depends(get_current_user)):
+    """可移动到的入口位置列表（含唯一上卷鞍座）。"""
+    return Response.ok(line_service.ENTRY_POSITIONS)
+
+
+@router.patch("/{plan_id}/move", response_model=Response[PlanOut])
+async def move_plan(plan_id: int, position: str = Query(...), db: AsyncSession = Depends(get_db), _ = Depends(get_current_user)):
+    """移动：把计划放到所选入口位置；放到上卷鞍座才触发生产联动。"""
+    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")
+    try:
+        await line_service.place_at_position(db, plan, position)
+    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))
+
+
 @router.patch("/{plan_id}/commit", response_model=Response[PlanOut])
 async def commit_producing(plan_id: int, db: AsyncSession = Depends(get_db), _ = Depends(get_current_user)):
     """投入生产：把鞍座上的计划置为生产中（兜底未实时变化的数据）。"""