feat(models): Task/Product 模型扩充 + 4个Alembic迁移

Task 模型:
- 新增 received_at, completed_at, reject_reason, is_rework
- 新增 TaskRecord 模型 (remark + images JSON)
- 状态常量 PENDING/WIP/COMPLETED/REJECTED/ARCHIVED
- 全部时间字段改用北京时间 (get_beijing_time)

Product 模型:
- order_id → 可选; 新增 external_serial, current_location_id
- 新增 material_name/spec_model/category/material_type 快照字段
- 新增 overall_status (备货/生产/测试/维修/在库)

迁移:
- b2c3: HEX计数器Sequence + external_serial + order_id nullable
- c3d4: material快照4列
- d4e5: overall_status
- e5f6: task_records表

backend/app/models/__init__.py

@@ -2,12 +2,13 @@
 from app.models.base import Base
 from app.models.production_order import ProductionOrder
 from app.models.product import Product
-from app.models.task import Task
+from app.models.task import Task, TaskRecord
 from app.models.task_log import TaskLog
 __all__ = [
     "Base",
     "ProductionOrder",
     "Product",
     "Task",
+    "TaskRecord",
     "TaskLog",
 ]