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/services/line_service.py

@@ -17,12 +17,45 @@ from app.models.production import ProductionRecord
 from app.models.downtime import DowntimeRecord
 from app.models.line_state import LineState
 
+# ── 入口位置 ──
+SADDLE_NAME = "上卷鞍座"        # 唯一会触发生产联动的位置
+ENTRY_POSITIONS = [
+    "1#上卷小车", "2#上卷小车",
+    "1#称重位", "2#称重位",
+    "1#地辊", "2#地辊",
+    "1#倒卷小车", "2#倒卷小车",
+    SADDLE_NAME,
+]
+
 # ── 仿真参数 ──
 TARGET_LENGTH_M = 2000.0       # 带头目标长度（生产完成阈值）
 SIM_SPEED_M_MIN = 600.0        # 仿真线速度 m/min（2000m ≈ 200s）
 DOWNTIME_THRESHOLD_S = 600      # 速度为0持续超过该秒数判定停机（10min）
 
 
+async def place_at_position(db: AsyncSession, plan: ProductionPlan, position: str):
+    """把计划放到任意入口位置；位置唯一占用。上卷鞍座单独触发生产联动。"""
+    if position not in ENTRY_POSITIONS:
+        raise ValueError("未知入口位置")
+    if position == SADDLE_NAME:
+        await move_to_saddle(db, plan)
+        return
+    # 普通位置：清除同位置上的其它计划，移出鞍座（若在）
+    res = await db.execute(
+        select(ProductionPlan).where(ProductionPlan.position == position, ProductionPlan.id != plan.id)
+    )
+    for o in res.scalars():
+        o.position = None
+    plan.on_saddle = 0
+    plan.run_started_at = None
+    plan.run_speed = 0
+    plan.run_length_m = 0
+    plan.position = position
+    if plan.status == "producing":
+        plan.status = "online"
+    await ensure_online(db)
+
+
 def _shift_of(dt: datetime) -> str:
     return "甲" if 8 <= dt.hour < 20 else "乙"
 
@@ -60,6 +93,7 @@ async def move_to_saddle(db: AsyncSession, plan: ProductionPlan):
     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")
     plan.on_saddle = 1
+    plan.position = SADDLE_NAME
     plan.saddle_at = datetime.now()
     plan.run_started_at = None
     plan.run_speed = 0
@@ -86,6 +120,7 @@ async def _produce(db: AsyncSession, plan: ProductionPlan):
     plan.status = "produced"
     plan.produced_at = now
     plan.on_saddle = 0
+    plan.position = None
     plan.run_speed = 0
     plan.run_length_m = TARGET_LENGTH_M