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

apps/test/features/todo/quadrant_drag_test.dart

@@ -27,6 +27,7 @@ void main() {
       () async {
         const todoId = 'todo-123';
         const targetPriority = 2;
+        const targetOrder = 0;
 
         when(
           () => mockClient.patch(any(), data: any(named: 'data')),
@@ -38,6 +39,7 @@ void main() {
               'owner_id': 'user-1',
               'title': 'Test Todo',
               'priority': targetPriority,
+              'order': targetOrder,
               'status': 'pending',
               'created_at': '2024-01-01T00:00:00Z',
               'updated_at': '2024-01-01T00:00:00Z',
@@ -48,13 +50,15 @@ void main() {
         final result = await todoApi.updateTodo(
           todoId,
           priority: targetPriority,
+          order: targetOrder,
         );
 
         expect(result.priority, targetPriority);
+        expect(result.order, targetOrder);
         verify(
           () => mockClient.patch(
             '/api/v1/todos/$todoId',
-            data: {'priority': targetPriority},
+            data: {'priority': targetPriority, 'order': targetOrder},
           ),
         ).called(1);
       },
@@ -67,7 +71,10 @@ void main() {
         () => mockClient.patch(any(), data: any(named: 'data')),
       ).thenThrow(Exception('Network error'));
 
-      expect(() => todoApi.updateTodo(todoId, priority: 2), throwsException);
+      expect(
+        () => todoApi.updateTodo(todoId, priority: 2, order: 0),
+        throwsException,
+      );
     });
   });
 
@@ -81,6 +88,7 @@ void main() {
             'owner_id': 'user-1',
             'title': 'Q1 Todo',
             'priority': 1,
+            'order': 1,
             'status': 'pending',
             'created_at': '2024-01-01T00:00:00Z',
             'updated_at': '2024-01-01T00:00:00Z',
@@ -88,8 +96,9 @@ void main() {
         ),
       );
 
-      final result = await todoApi.updateTodo('todo-1', priority: 1);
+      final result = await todoApi.updateTodo('todo-1', priority: 1, order: 1);
       expect(result.priority, 1);
+      expect(result.order, 1);
     });
 
     test('priority 2 = important not urgent (Q3)', () async {
@@ -101,6 +110,7 @@ void main() {
             'owner_id': 'user-1',
             'title': 'Q3 Todo',
             'priority': 2,
+            'order': 2,
             'status': 'pending',
             'created_at': '2024-01-01T00:00:00Z',
             'updated_at': '2024-01-01T00:00:00Z',
@@ -108,8 +118,9 @@ void main() {
         ),
       );
 
-      final result = await todoApi.updateTodo('todo-2', priority: 2);
+      final result = await todoApi.updateTodo('todo-2', priority: 2, order: 2);
       expect(result.priority, 2);
+      expect(result.order, 2);
     });
 
     test('priority 3 = urgent not important (Q2)', () async {
@@ -121,6 +132,7 @@ void main() {
             'owner_id': 'user-1',
             'title': 'Q2 Todo',
             'priority': 3,
+            'order': 0,
             'status': 'pending',
             'created_at': '2024-01-01T00:00:00Z',
             'updated_at': '2024-01-01T00:00:00Z',
@@ -128,8 +140,37 @@ void main() {
         ),
       );
 
-      final result = await todoApi.updateTodo('todo-3', priority: 3);
+      final result = await todoApi.updateTodo('todo-3', priority: 3, order: 0);
       expect(result.priority, 3);
+      expect(result.order, 0);
+    });
+  });
+
+  group('TodoApi.reorderTodos', () {
+    test('calls batch reorder endpoint once', () async {
+      when(() => mockClient.patch(any(), data: any(named: 'data'))).thenAnswer(
+        (_) async => Response(
+          requestOptions: RequestOptions(path: '/api/v1/todos/reorder'),
+          data: {},
+        ),
+      );
+
+      await todoApi.reorderTodos(const [
+        TodoReorderItemPayload(id: 'todo-1', priority: 1, order: 0),
+        TodoReorderItemPayload(id: 'todo-2', priority: 1, order: 1),
+      ]);
+
+      verify(
+        () => mockClient.patch(
+          '/api/v1/todos/reorder',
+          data: {
+            'items': [
+              {'id': 'todo-1', 'priority': 1, 'order': 0},
+              {'id': 'todo-2', 'priority': 1, 'order': 1},
+            ],
+          },
+        ),
+      ).called(1);
     });
   });
 }