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; }
+    }
+}

Entities/DTO/KeyPoint/KeyPointDto.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public record KeyPointDto(Guid Id, string Key, Guid LessonId);
+}

Entities/DTO/KeyPoint/KeyPointResponseDto.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace Entities.DTO
+{
+    public record KeyPointResponseDto
+    {
+        public Guid Id { get; init; }
+        public string Key { get; init; } = string.Empty;
+        public Guid LessonID { get; init; }
+        public DateTime CreatedAt { get; init; }
+        public DateTime? UpdatedAt { get; init; }
+    }
+}

Entities/DTO/KeyPoint/UpdateKeyPointDto.cs

@@ -0,0 +1,17 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public record UpdateKeyPointDto
+    {
+        [Required]
+        public Guid Id { get; init; }
+
+        [Required]
+        [StringLength(255)]
+        public string Key { get; init; } = string.Empty;
+
+        [Required]
+        public Guid LessonID { get; init; }
+    }
+}