feat(models): 扩充 Task 和 Product 数据库模型

Task 模型新增:
- received_at: DateTime (操作员确认接收时间)
- completed_at: DateTime (任务完工转交时间)
- reject_reason: String(500) (驳回原因)
- is_rework: Boolean (是否为返工任务，默认 False)
- 状态常量: PENDING/WIP/COMPLETED/REJECTED/ARCHIVED
- 默认状态由 'pending' 升为大写 'PENDING'

Product 模型新增:
- current_location_id: String(64) (当前持有者ID 或 'virtual_warehouse')

backend/app/models/product.py

@@ -33,6 +33,11 @@ class Product(Base):
         UUID(as_uuid=True), ForeignKey("products.id"), nullable=True, comment="父产品ID",
     )
 
+    # ---- 当前位置追踪（逻辑外键→用户ID 或 'virtual_warehouse'） ----
+    current_location_id: Mapped[str | None] = mapped_column(
+        String(64), nullable=True, comment="当前持有者ID 或 'virtual_warehouse'(仓库)",
+    )
+
     status: Mapped[str] = mapped_column(
         String(50), nullable=False, default="pending", comment="产品状态",
     )