fix(entry): 恢复入口跟踪全部设备工位，修复鞍座联动状态竞争

- 还原入口跟踪原有设备网格(2行)：上卷小车/称重位/地辊/倒卷小车，仅上卷鞍座保留一个
- 移动可在所有入口设备间进行，仅「上卷鞍座」进入生产环节
- 修复 ensure_online 误将鞍座暂存计划回退导致卡死：移动到鞍座直接置生产中，
  ensure_online 排除 on_saddle 计划

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

backend/app/services/line_service.py

@@ -66,8 +66,10 @@ async def _saddle_plan(db: AsyncSession):
 
 
 async def ensure_online(db: AsyncSession):
-    """保证恰好一条 online（队首，最早录入的 ready）。"""
-    res = await db.execute(select(ProductionPlan).where(ProductionPlan.status == "online"))
+    """保证恰好一条 online（队首，最早录入的 ready）。鞍座上的计划不参与队列。"""
+    res = await db.execute(
+        select(ProductionPlan).where(ProductionPlan.status == "online", ProductionPlan.on_saddle != 1)
+    )
     online = list(res.scalars())
     if len(online) > 1:
         # 仅保留最早的一条为 online，其余回退 ready
@@ -88,25 +90,27 @@ async def ensure_online(db: AsyncSession):
 
 
 async def move_to_saddle(db: AsyncSession, plan: ProductionPlan):
-    """把在线计划移动到上卷鞍座（staged，等待速度/投入生产）。"""
+    """把计划移动到上卷鞍座：上卷即获得速度 → 生产中。"""
     occupied = await _saddle_plan(db)
     if occupied and occupied.id != plan.id:
         raise ValueError("上卷鞍座已被占用，请等待当前钢卷生产完成")
+    now = datetime.now()
     plan.on_saddle = 1
     plan.position = SADDLE_NAME
-    plan.saddle_at = datetime.now()
-    plan.run_started_at = None
-    plan.run_speed = 0
-    plan.run_length_m = 0
-    if plan.status not in ("producing", "produced"):
-        plan.status = "online"
+    plan.saddle_at = now
+    if plan.status != "produced":
+        plan.run_started_at = now
+        plan.run_speed = SIM_SPEED_M_MIN
+        plan.run_length_m = 0
+        plan.status = "producing"
     await ensure_online(db)
 
 
 async def commit_plan(db: AsyncSession, plan: ProductionPlan):
-    """投入生产：鞍座计划有速度 → 生产中。"""
+    """投入生产（兜底）：鞍座计划置为生产中。"""
     if plan.on_saddle != 1:
         await move_to_saddle(db, plan)
+        return
     if plan.run_started_at is None:
         plan.run_started_at = datetime.now()
     plan.run_speed = SIM_SPEED_M_MIN
@@ -166,17 +170,11 @@ async def _produce(db: AsyncSession, plan: ProductionPlan):
 
 
 async def advance_saddle(db: AsyncSession):
-    """推进鞍座计划：staged 自动获得速度 → 生产中；累计长度到 2000m → 完成。"""
+    """推进鞍座计划：累计带头长度到 2000m → 生产完成。"""
     plan = await _saddle_plan(db)
     if not plan:
         return
     now = datetime.now()
-    # staged（在线且在鞍座）自动获得速度，模拟 PLC 速度信号
-    if plan.status == "online" and plan.run_started_at is None:
-        plan.run_started_at = now
-        plan.run_speed = SIM_SPEED_M_MIN
-        plan.status = "producing"
-        await ensure_online(db)
     if plan.status == "producing" and plan.run_started_at:
         elapsed = (now - plan.run_started_at).total_seconds()
         plan.run_length_m = min(TARGET_LENGTH_M, (plan.run_speed or 0) / 60.0 * elapsed)