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/product.py

@@ -1,11 +1,12 @@
 """产品模型"""
 import uuid
-from datetime import datetime, timezone
+from datetime import datetime
 from sqlalchemy import String, DateTime, ForeignKey
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.models.base import Base
+from app.core.time_utils import get_beijing_time
 
 
 class Product(Base):
@@ -18,15 +19,33 @@ class Product(Base):
         String(16), unique=True, index=True, nullable=False, comment="产品序列号(16位)",
     )
 
-    # ---- 物理外键（关联本库 production_orders） ----
-    order_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("production_orders.id"), nullable=False, comment="所属订单ID",
+    # ---- 物理外键（关联本库 production_orders）— 可选 ----
+    order_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("production_orders.id"), nullable=True, comment="所属订单ID(可选)",
+    )
+
+    # ---- 外部序列号（用户自定义，可选） ----
+    external_serial: Mapped[str | None] = mapped_column(
+        String(64), nullable=True, comment="用户自定义产品序列号(可选)",
     )
 
     # ---- 逻辑外键（关联老系统物料表，仅存储 ID，无物理约束） ----
     material_id: Mapped[str | None] = mapped_column(
         String(64), nullable=True, comment="物料ID(逻辑外键→老系统)",
     )
+    # ---- MOM 物料快照字段（创产品时写入，防止老系统数据变动影响标签） ----
+    material_name: Mapped[str | None] = mapped_column(
+        String(255), nullable=True, comment="物料名称快照",
+    )
+    spec_model: Mapped[str | None] = mapped_column(
+        String(255), nullable=True, comment="规格型号快照",
+    )
+    category: Mapped[str | None] = mapped_column(
+        String(255), nullable=True, comment="物料分类快照",
+    )
+    material_type: Mapped[str | None] = mapped_column(
+        String(100), nullable=True, comment="物料类型快照",
+    )
 
     # ---- 物理外键（自引用：父产品） ----
     parent_product_id: Mapped[uuid.UUID | None] = mapped_column(
@@ -41,8 +60,12 @@ class Product(Base):
     status: Mapped[str] = mapped_column(
         String(50), nullable=False, default="pending", comment="产品状态",
     )
+    # 宏观流转状态 — 首次扫码时强制设定：备货/生产/测试/维修/在库
+    overall_status: Mapped[str | None] = mapped_column(
+        String(20), nullable=True, comment="宏观状态: 备货/生产/测试/维修/在库",
+    )
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), comment="创建时间",
+        DateTime(timezone=True), default=get_beijing_time, comment="创建时间",
     )
 
     # ---- 关系 ----