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

Entities/DTO/Grade/CreateGradeDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public record CreateGradeDto
+    {
+        public Guid SchoolId { get; init; }
+
+        [Required(ErrorMessage = "年级名称是必填项。")]
+        [StringLength(20, ErrorMessage = "年级名称不能超过 20 个字符。")]
+        public string GradeName { get; init; }
+
+        [Required(ErrorMessage = "年级级别是必填项。")]
+        public byte GradeLevel { get; init; }
+    }
+}

Entities/DTO/Grade/GradeDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class GradeDto
+    {
+        public Guid Id { get; set; }
+
+        public Guid SchoolId { get; set; }
+
+        [Required(ErrorMessage = "年级名称是必填项。")]
+        [StringLength(20, ErrorMessage = "年级名称不能超过 20 个字符。")]
+        public string GradeName { get; set; }
+
+        [Required(ErrorMessage = "年级级别是必填项。")]
+        public byte GradeLevel { get; set; }
+    }
+}

Entities/DTO/Grade/GradeResponseDto.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class GradeResponseDto
+    {
+        public int Id { get; set; }
+        public int SchoolId { get; set; }
+        public string SchoolName { get; set; }
+        public string GradeName { get; set; }
+        public int GradeLevel { get; set; }
+
+        // Navigation properties for response
+        public ICollection<ClassDto> Responses { get; set; }
+    }
+}

Entities/DTO/Grade/UpdateGradeDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class UpdateGradeDto
+    {
+        [Required(ErrorMessage = "年级ID是必填项。")]
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "学校ID是必填项。")]
+        public Guid SchoolId { get; set; }
+
+        [Required(ErrorMessage = "年级名称是必填项。")]
+        [StringLength(20, ErrorMessage = "年级名称不能超过 20 个字符。")]
+        public string GradeName { get; set; }
+
+        [Required(ErrorMessage = "年级级别是必填项。")]
+        public byte GradeLevel { get; set; }
+    }
+}