fix: make admin media uploads reliable

src/features/admin/courses/schema/index.js

@@ -2,7 +2,8 @@ import { array, boolean, mixed, number, object, string } from 'yup'
 
 export const courseSchema = object().shape({
   title: string().required().min(3).max(255),
-  teacherId: mixed().required(),
+  // The backend intentionally allows courses without an assigned teacher.
+  teacherId: mixed().nullable().notRequired(),
   capacity: number()
     .typeError('ظرفیت باید به‌صورت عدد وارد شود')
     .required()