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

Entities/DTO/Class/ClassCreateDto.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO.Class
+{
+	public record ClassCreateDto
+	{
+		public CreateGradeDto Grade { 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 ClassName { get; init; }
+
+		[Required(ErrorMessage = "年级级别是必填项。")]
+		public byte Index { get; init; }
+	}
+}

Entities/DTO/Class/ClassDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class ClassDto
+    {
+        public Guid Id { get; set; }
+
+        public Guid GradeId { get; set; }
+
+        public GradeDto Grade { get; set; }
+
+        [Required(ErrorMessage = "班级名称是必填项。")]
+        [StringLength(30, ErrorMessage = "班级名称不能超过 30 个字符。")]
+        public string ClassName { get; set; }
+
+        public byte Index { get; set; }
+
+        public Guid? HeadTeacherId { get; set; }
+
+        public DateTime CreatedAt { get; set; }
+        public DateTime UpdatedAt { get; set; }
+    }
+}

Entities/DTO/Class/ClassResponseDto.cs

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+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class ClassResponseDto
+    {
+        public Guid Id { get; set; }
+        public Guid GradeId { get; set; }
+        public string GradeName { get; set; }
+        public string ClassName { get; set; }
+        public Guid? HeadTeacherId { get; set; }
+        public string HeadTeacherName { get; set; }
+        public DateTime CreatedAt { get; set; }
+        public DateTime UpdatedAt { get; set; }
+        public bool IsDeleted { get; set; }
+
+        // Navigation properties for response
+        public ICollection<ClassUserDto> ClassStudents { get; set; }
+    }
+}

Entities/DTO/Class/ClassUserDto.cs

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+using System;
+
+namespace Entities.DTO
+{
+    public class ClassUserDto
+    {
+        public Guid StudentId { get; set; }
+        public string? Email { get; set; }
+        public string? DisplayName { get; set; }
+        public string? PhoneNumber { get; set; }
+        public string? Address { get; set; }
+        public bool TwoFactorEnabled { get; set; }
+        public bool EmailConfirmed { get; set; }
+    }
+}

Entities/DTO/Class/ClassValidDto.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public record ClassValidDto(string SchoolName, byte Grade, byte Class);
+}

Entities/DTO/Class/CreateClassDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class CreateClassDto
+    {
+        [Required(ErrorMessage = "年级ID是必填项。")]
+        public Guid GradeId { get; set; }
+
+        [Required(ErrorMessage = "班级名称是必填项。")]
+        [StringLength(30, ErrorMessage = "班级名称不能超过 30 个字符。")]
+        public string ClassName { get; set; }
+
+        public byte Index { get; set; }
+
+        public Guid? HeadTeacherId { get; set; }
+    }
+}

Entities/DTO/Class/RegisterUserToClassDto.cs

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+using Entities.Contracts;
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class RegisterUserToClassDto
+    {
+
+        public Guid UserId { get; set; }
+
+        public Guid ClassId { get; set; }
+
+        public UserRoles Role { get; set; } = UserRoles.Student; 
+
+        public Guid? SubjectArea { get; set; }
+    }
+}

Entities/DTO/Class/UpdateClassDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class UpdateClassDto
+    {
+        [Required(ErrorMessage = "班级ID是必填项。")]
+        public Guid ClassId { get; set; }
+
+        [Required(ErrorMessage = "年级ID是必填项。")]
+        public Guid GradeId { get; set; }
+
+        [Required(ErrorMessage = "班级名称是必填项。")]
+        [StringLength(30, ErrorMessage = "班级名称不能超过 30 个字符。")]
+        public string ClassName { get; set; }
+
+        public Guid? HeadTeacherId { get; set; }
+    }
+}