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

Entities/DTO/Subject/CreateSubjectDto.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class CreateSubjectDto
+    {
+        [Required(ErrorMessage = "科目名称是必填项。")]
+        [StringLength(20, ErrorMessage = "科目名称不能超过 20 个字符。")]
+        public string SubjectName { get; set; }
+
+        public string Description { get; set; }
+    }
+}

Entities/DTO/Subject/SubjectDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class SubjectDto
+    {
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "科目名称是必填项。")]
+        [StringLength(20, ErrorMessage = "科目名称不能超过 20 个字符。")]
+        public string Name { get; set; }
+
+        public string Description { get; set; }
+    }
+}

Entities/DTO/Subject/SubjectResponseDto.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class SubjectResponseDto
+    {
+        public Guid Id { get; set; }
+        public string Name { get; set; }
+        public string Description { get; set; }
+
+        // Navigation properties for response
+        public int QuestionTypeCount { get; set; }
+        public int QuestionCount { get; set; }
+        public int TeacherCount { get; set; }
+        public List<string> QuestionTypeNames { get; set; } = new List<string>();
+        public List<string> TeacherNames { get; set; } = new List<string>();
+    }
+}

Entities/DTO/Subject/UpdateSubjectDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class UpdateSubjectDto
+    {
+        [Required(ErrorMessage = "科目ID是必填项。")]
+        public Guid SubjectId { get; set; }
+
+        [Required(ErrorMessage = "科目名称是必填项。")]
+        [StringLength(20, ErrorMessage = "科目名称不能超过 20 个字符。")]
+        public string SubjectName { get; set; }
+
+        public string Description { get; set; }
+    }
+}