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/task.py

@@ -1,11 +1,12 @@
 """任务模型 — 支持无限嵌套、任务裂变分支、返工闭环"""
 import uuid
-from datetime import datetime, timezone
+from datetime import datetime
 from sqlalchemy import String, DateTime, Boolean, 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
 
 # 任务状态常量
 TASK_STATUS_PENDING = "PENDING"       # 待接收
@@ -52,7 +53,7 @@ class Task(Base):
 
     # ---- 时间追踪 ----
     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",
     )
     received_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True, comment="操作员确认接收时间",
@@ -77,6 +78,39 @@ class Task(Base):
     child_tasks: Mapped[list["Task"]] = relationship(
         "Task", back_populates="parent_task", lazy="selectin",
     )
+    records: Mapped[list["TaskRecord"]] = relationship(
+        "TaskRecord", back_populates="task", lazy="selectin", cascade="all, delete-orphan",
+    )
 
     def __repr__(self) -> str:
         return f"<Task {self.task_name}>"
+
+
+# ============================================================
+# 任务进度记录 — 随时备注/传图
+# ============================================================
+
+class TaskRecord(Base):
+    __tablename__ = "task_records"
+
+    id: Mapped[int] = mapped_column(
+        primary_key=True, autoincrement=True,
+    )
+    task_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tasks.id"), nullable=False, index=True, comment="所属任务ID",
+    )
+    remark: Mapped[str | None] = mapped_column(
+        String(2000), nullable=True, comment="备注文本",
+    )
+    images: Mapped[str | None] = mapped_column(
+        String(4000), nullable=True, comment="图片URL列表(JSON字符串)",
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=get_beijing_time, comment="记录时间",
+    )
+
+    # ---- 关系 ----
+    task: Mapped["Task"] = relationship("Task", back_populates="records")
+
+    def __repr__(self) -> str:
+        return f"<TaskRecord {self.id} @ {self.created_at}>"