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

Entities/Contracts/ExamAttachment.cs

@@ -0,0 +1,47 @@
+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 System.Diagnostics.CodeAnalysis;
+
+namespace Entities.Contracts
+{
+	[Table("exam_attachments")]
+	public class ExamAttachment
+	{
+		[Key]
+		[Column("id")]
+		public Guid Id { get; set; }
+
+		[Required]
+		[Column("exam_id")]
+		public Guid ExamId { get; set; }
+		[ForeignKey(nameof(ExamId))]
+		public virtual Exam Exam { get; set; }
+
+		[Required]
+		[Column("file_path")]
+		[StringLength(255)]
+		public string FilePath { get; set; }
+
+		[Required]
+		[Column("file_name")]
+		[StringLength(255)]
+		public string FileName { get; set; }
+
+		[Column("uploaded_at")]
+		public DateTime UploadedAt { get; set; }
+
+		[Column("deleted")]
+		public bool IsDeleted { get; set; }
+
+		public ExamAttachment()
+		{
+			Id = Guid.NewGuid();
+		}
+
+	}
+}