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

TechHelper.Server/Context/Configuration/SubmissionConfiguration.cs

@@ -21,7 +21,7 @@ namespace TechHelper.Context.Configuration
 				.HasColumnName("id");
 
 			// AssignmentId
-			builder.Property(s => s.AssignmentId)
+			builder.Property(s => s.ExamId)
 				.HasColumnName("assignment_id")
 				.IsRequired();
 
@@ -67,9 +67,9 @@ namespace TechHelper.Context.Configuration
 
 
 
-			builder.HasOne(s => s.Assignment)         // 当前 Submission 有一个 Assignment
+			builder.HasOne(s => s.Exam)         // 当前 Submission 有一个 Assignment
 				.WithMany(a => a.Submissions)         // 那个 Assignment 可以有多个 Submission
-				.HasForeignKey(s => s.AssignmentId)   // 外键是 Submission.AssignmentId
+				.HasForeignKey(s => s.ExamId)   // 外键是 Submission.AssignmentId
 				.OnDelete(DeleteBehavior.Cascade);    // 当 Assignment 被删除时，相关的 Submission 也级联删除。