feat(services): 重写核心业务逻辑 — 接收/驳回返工/裂变转交

task_service 新增 3 个核心业务函数:
- receive_task(): PENDING→WIP, 记录 received_at, 仅 PENDING 状态可接收
- reject_task(): →REJECTED, 记录 reject_reason+completed_at,
  防呆闭环: 自动查找父任务负责人, 创建 is_rework=True 的返工任务
- transfer_task(): →COMPLETED, 记录 completed_at,
  裂变转交: 遍历 next_assignees 批量创建 PENDING 任务,
  多路裂变(>1)时子任务挂载形成树状分支,
  virtual_warehouse 入库逻辑更新 Product.current_location_id
- complete_task(): 保留兼容, 同步使用新状态常量 TASK_STATUS_*

product_service 重构:
- get_product_by_serial(): 新增 task_tree 返回完整递归任务树
- _load_task_tree(): 递归加载产品关联的所有任务嵌套关系

backend/app/services/__init__.py

@@ -0,0 +1,40 @@
+"""业务逻辑层"""
+from app.services.product_service import (
+    get_product_by_serial,
+    get_product,
+    create_product,
+    update_product,
+    get_all_products,
+)
+from app.services.task_service import (
+    get_task,
+    get_top_level_tasks,
+    create_task,
+    update_task,
+    complete_task,
+    receive_task,
+    reject_task,
+    transfer_task,
+    create_subtask,
+    get_all_tasks,
+)
+
+__all__ = [
+    # Product
+    "get_product_by_serial",
+    "get_product",
+    "create_product",
+    "update_product",
+    "get_all_products",
+    # Task
+    "get_task",
+    "get_top_level_tasks",
+    "create_task",
+    "update_task",
+    "complete_task",
+    "receive_task",
+    "reject_task",
+    "transfer_task",
+    "create_subtask",
+    "get_all_tasks",
+]