feat: add dedicated thumbnail upload endpoint for courses with old file cleanup and presigned URL generation.

Backend/src/controllers/CoursesInstructorController.ts

@@ -111,6 +111,29 @@ export class CoursesInstructorController {
         return await CoursesInstructorService.createCourse(value, decoded.id, thumbnail);
     }
 
+    /**
+     * อัปโหลดรูป thumbnail ของคอร์ส
+     * Upload course thumbnail image
+     * @param courseId - รหัสคอร์ส / Course ID
+     * @param file - ไฟล์รูปภาพ / Image file
+     */
+    @Post('{courseId}/thumbnail')
+    @Security('jwt', ['instructor'])
+    @SuccessResponse('200', 'Thumbnail uploaded successfully')
+    @Response('401', 'Invalid or expired token')
+    @Response('400', 'Validation error')
+    public async uploadThumbnail(
+        @Request() request: any,
+        @Path() courseId: number,
+        @UploadedFile() file: Express.Multer.File
+    ): Promise<{ code: number; message: string; data: { course_id: number; thumbnail_url: string } }> {
+        const token = request.headers.authorization?.replace('Bearer ', '');
+        if (!token) throw new ValidationError('No token provided');
+        if (!file.mimetype?.startsWith('image/')) throw new ValidationError('Only image files are allowed');
+
+        return await CoursesInstructorService.uploadThumbnail(token, courseId, file);
+    }
+
     /**
      * ลบคอร์ส (เฉพาะผู้สอนหลักเท่านั้น)
      * Delete a course (only primary instructor can delete)