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

backend/tests/unit/v1/todo/test_schemas.py

@@ -0,0 +1,65 @@
+from pydantic import ValidationError
+
+from v1.todo.schemas import TodoCreate, TodoReorderRequest, TodoResponse, TodoUpdate
+
+
+def test_todo_create_accepts_zero_based_order() -> None:
+    request = TodoCreate.model_validate({"title": "Test", "priority": 1, "order": 0})
+
+    assert request.order == 0
+
+
+def test_todo_create_rejects_due_at_field() -> None:
+    try:
+        TodoCreate.model_validate(
+            {
+                "title": "Test",
+                "priority": 1,
+                "order": 0,
+                "due_at": "2026-01-01T00:00:00Z",
+            }
+        )
+    except ValidationError:
+        return
+
+    raise AssertionError("TodoCreate should reject due_at")
+
+
+def test_todo_update_rejects_negative_order() -> None:
+    try:
+        TodoUpdate.model_validate({"order": -1})
+    except ValidationError:
+        return
+
+    raise AssertionError("TodoUpdate should reject negative order")
+
+
+def test_todo_response_contains_order_without_due_at() -> None:
+    payload = {
+        "id": "9d72aa0f-e2d5-4de3-9b2d-4609f53376c0",
+        "owner_id": "2f5b38fb-cb6f-44ef-8824-e7978f644bc2",
+        "title": "Test",
+        "description": None,
+        "priority": 1,
+        "order": 0,
+        "status": "pending",
+        "completed_at": None,
+        "created_at": "2026-03-20T00:00:00Z",
+        "updated_at": "2026-03-20T00:00:00Z",
+        "schedule_items": [],
+    }
+
+    response = TodoResponse.model_validate(payload)
+    dumped = response.model_dump()
+
+    assert dumped["order"] == 0
+    assert "due_at" not in dumped
+
+
+def test_todo_reorder_request_requires_non_empty_items() -> None:
+    try:
+        TodoReorderRequest(items=[])
+    except ValidationError:
+        return
+
+    raise AssertionError("TodoReorderRequest should reject empty items")