重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/KeyPoint/CreateKeyPointDto.cs

@@ -0,0 +1,14 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public record CreateKeyPointDto
+    {
+        [Required]
+        [StringLength(255)]
+        public string Key { get; init; } = string.Empty;
+
+        [Required]
+        public Guid LessonID { get; init; }
+    }
+}