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

Entities/Contracts/Subject.cs

@@ -0,0 +1,36 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Entities.Contracts
+{
+    [Table("subjects")]
+    public class Subject
+    {
+        [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(QuestionType.Subject))]
+        public virtual IEnumerable<QuestionType> QuestionTypes { get; set; }
+
+        [InverseProperty(nameof(Question.Subject))]
+        public virtual IEnumerable<Question> Questions { get; set; }
+
+        [InverseProperty(nameof(User.TeachSubject))]
+        public virtual IEnumerable<User> SubjectTeachers { get; set; }
+        public Subject()
+        {
+            Id = Guid.NewGuid();
+            QuestionTypes = new HashSet<QuestionType>();
+			Questions = new HashSet<Question>();
+		}
+    }
+}