refactor(todo): 移除 due_at 字段，改用 order 字段管理象限内顺序

backend/src/models/todos.py

@@ -26,7 +26,7 @@ class TodoPriority(int, Enum):
 
 class Todo(TimestampMixin, SoftDeleteMixin, Base):
     __tablename__: str = "todos"
-    __table_args__ = {"extend_existing": True}
+    __table_args__: dict[str, bool] = {"extend_existing": True}
 
     id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
@@ -43,9 +43,10 @@ class Todo(TimestampMixin, SoftDeleteMixin, Base):
         String(1000),
         nullable=True,
     )
-    due_at: Mapped[datetime | None] = mapped_column(
-        DateTime(timezone=True),
-        nullable=True,
+    order: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        default=0,
     )
     priority: Mapped[int] = mapped_column(
         Integer,