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

Entities/Contracts/Grade.cs

@@ -10,24 +10,25 @@ namespace Entities.Contracts
     {
         [Key]
         [Column("grade_id")]
-        public int GradeId { get; set; }
+        public Guid Id { get; set; }
 
         [Column("school_id")]
-        public int SchoolId { get; set; }
-        public School School { get; set; }
+        public Guid SchoolId { get; set; }
+        public virtual School School { get; set; }
 
         [Column("grade_name")]
         [MaxLength(20)]
         public string GradeName { get; set; }
 
         [Column("grade_level")]
-        public int GradeLevel { get; set; }
+        public byte GradeLevel { get; set; }
 
-        // Navigation Properties
-        public ICollection<Class> Classes { get; set; }
+        [InverseProperty(nameof(Class.Grade))]  
+        public virtual ICollection<Class> Classes { get; set; }
 
         public Grade()
         {
+            Id = Guid.NewGuid();
             Classes = new HashSet<Class>();
         }
     }