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

Entities/Contracts/ClassUser.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore;
+
+namespace Entities.Contracts
+{
+	/// <summary>
+	/// 班级用户关联实体类
+	/// 表示用户与班级之间的多对多关系
+	/// </summary>
+	[Table("class_user")]
+    [PrimaryKey(nameof(ClassId), nameof(UserId))]
+    public class ClassUser
+    {
+        /// <summary>
+        /// 班级ID，复合主键的一部分，外键
+        /// 关联到Class表中的班级
+        /// </summary>
+        [Key]
+        [Column("class_id", Order = 0)]
+        public Guid ClassId { get; set; }
+        [ForeignKey(nameof(ClassId))]
+		public virtual Class Class { get; set; }
+		/// <summary>
+		/// 用户ID，复合主键的一部分，外键
+		/// 关联到User表中的用户用户
+		/// </summary>
+		[Key]
+        [Column("student_id", Order = 1)]
+        public Guid UserId { get; set; }
+		[ForeignKey(nameof(UserId))]
+		public virtual User User { get; set; }
+
+		/// <summary>
+		/// 入学日期
+		/// 记录用户加入该班级的日期
+		/// </summary>
+		[Column("enrollment_date")]
+        public DateTime EnrollmentDate { get; set; }
+
+        /// <summary>
+        /// 是否已删除
+        /// 软删除标记，true表示已删除
+        /// </summary>
+        [Column("deleted")]
+        public bool IsDeleted { get; set; }
+
+
+
+    }
+}