perf: CTE任务树加载器 + MOM跨库查询缓存

消除两个核心N+1性能瓶颈：

1. CTE任务树加载器 (task_tree_loader.py)
   - PostgreSQL Recursive CTE一次性加载完整任务树
   - 无论树深度多大，仅2条SQL（CTE + records selectinload）
   - set_committed_value安全注入，避免Session脏数据
   - 修复add_task_record双重加载问题
   - 移除get_all_tasks中冗余的selectinload(child_tasks)

2. MOM跨库查询缓存 (mom_cache.py)
   - 零依赖TTL内存缓存（threading.RLock + time.monotonic）
   - 参数化ANY(:user_ids)替代OR拼接LIKE（防注入）
   - get_all_products中3次调用共享缓存，2h TTL内零跨库查询

backend/app/services/product_service.py

@@ -46,35 +46,10 @@ def _task_to_response(task: Task) -> TaskResponse:
 
 
 async def _load_task_tree(db: AsyncSession, product_id: uuid.UUID) -> list[TaskResponse]:
-    """递归加载产品下的完整任务树"""
-    # 先取顶层任务
-    result = await db.execute(
-        select(Task)
-        .options(selectinload(Task.child_tasks), selectinload(Task.records))
-        .where(
-            Task.product_id == product_id,
-            Task.parent_task_id.is_(None),
-        )
-        .order_by(Task.created_at)
-    )
-    top_tasks = result.scalars().all()
-
-    # 递归加载每层子任务
-    async def _load_children(t: Task):
-        for child in t.child_tasks:
-            child_result = await db.execute(
-                select(Task)
-                .options(selectinload(Task.child_tasks), selectinload(Task.records))
-                .where(Task.id == child.id)
-            )
-            refreshed = child_result.scalar_one()
-            t.child_tasks[t.child_tasks.index(child)] = refreshed
-            await _load_children(refreshed)
-
-    for task in top_tasks:
-        await _load_children(task)
-
-    return [_task_to_response(t) for t in top_tasks]
+    """使用 PostgreSQL Recursive CTE 一次性加载产品下完整任务树（消除 N+1）"""
+    from app.services.task_tree_loader import load_task_trees_by_product
+    tasks = await load_task_trees_by_product(db, product_id)
+    return [_task_to_response(t) for t in tasks]
 
 
 async def get_product_by_serial(db: AsyncSession, serial_number: str) -> ProductScanResponse:
@@ -344,32 +319,9 @@ async def update_overall_status(
 
 
 def _lookup_display_names(location_ids: list[str]) -> dict[str, str]:
-    """批量查询 MOM sys_user，将 username 映射为真实姓名"""
-    if not location_ids:
-        return {}
-    from app.core.mom_database import MomSessionLocal
-    from sqlalchemy import text
-    db = MomSessionLocal()
-    try:
-        # 过滤掉特殊值
-        real_ids = [uid for uid in location_ids if uid and uid != "virtual_warehouse"]
-        if not real_ids:
-            return {}
-        # 用 LIKE 模糊匹配批量查出
-        conditions = " OR ".join([f"username LIKE '%/{uid}'" for uid in real_ids])
-        result = db.execute(
-            text(f"SELECT username, SPLIT_PART(username, '/', 1) as display_name FROM sys_user WHERE {conditions}")
-        )
-        mapping = {}
-        for row in result:
-            full_username = row[0]
-            display_name = row[1]
-            # 从 full_username 末尾提取短用户名: "张三/zhangsan01" → "zhangsan01"
-            short = full_username.split("/")[-1] if "/" in full_username else full_username
-            mapping[short] = display_name
-        return mapping
-    finally:
-        db.close()
+    """批量查询 MOM sys_user，将 username 映射为真实姓名（带 2h TTL 缓存）"""
+    from app.services.mom_cache import get_display_names
+    return get_display_names(location_ids)
 
 
 async def get_all_products(