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

Entities/Contracts/ExamType.cs

@@ -0,0 +1,30 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Entities.Contracts
+{
+    [Table("exam_type")]
+    public class ExamType
+	{
+        [Key]
+        [Column("id")]
+        public Guid Id { get; set; }
+
+        [Column("name")]
+        [MaxLength(20)]
+        public string Name { get; set; }
+
+        [Column("description")]
+        public string Description { get; set; }
+
+        [InverseProperty(nameof(Exam.ExamType))]
+        public virtual ICollection<Exam> Exams { get; set; }
+
+        public ExamType()
+        {
+			Id = Guid.NewGuid();
+			Exams = new HashSet<Exam>();
+        }
+    }
+}