fix: nullable|integer — system controllers accept null/empty for optional integer fields

backend/app/Http/Controllers/Admin/System/DepartmentController.php

@@ -22,9 +22,9 @@ class DepartmentController extends Controller
     public function store(Request $request): JsonResponse
     {
         $data = $request->validate([
-            'parent_id' => 'integer',
+            'parent_id' => 'nullable|integer',
             'name' => 'required|string|max:50',
-            'sort' => 'integer',
+            'sort' => 'nullable|integer',
             'status' => 'in:0,1',
         ]);
 
@@ -36,9 +36,9 @@ class DepartmentController extends Controller
     public function update(Request $request, Department $department): JsonResponse
     {
         $data = $request->validate([
-            'parent_id' => 'integer',
+            'parent_id' => 'nullable|integer',
             'name' => 'string|max:50',
-            'sort' => 'integer',
+            'sort' => 'nullable|integer',
             'status' => 'in:0,1',
         ]);