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

Entities/Contracts/QuestionType.cs

@@ -0,0 +1,40 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Entities.Contracts
+{
+    [Table("question_types")]
+    public class QuestionType
+    {
+        [Key]
+        [Column("id")]
+        public Guid Id { get; set; }
+
+        [Column("name")]
+        [MaxLength(20)]
+        public string Name { get; set; }
+
+        [Column("subject_id")]
+        public Guid SubjectId { get; set; }
+        [ForeignKey(nameof(SubjectId))]
+        public virtual Subject Subject { get; set; }
+
+        [Column("description")]
+        public string Description { get; set; }
+
+        [Column("score_rule")]
+        [MaxLength(20)]
+        public string ScoreRule { get; set; }
+
+        [InverseProperty(nameof(Question.QuestionType))]
+        public virtual IEnumerable<Question> Questions { get; set; }
+
+        public QuestionType()
+        {
+            Id = Guid.NewGuid();
+
+			Questions = new HashSet<Question>();
+		}
+	}
+}