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

Entities/DTO/School/CreateSchoolDto.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class CreateSchoolDto
+    {
+        [Required(ErrorMessage = "学校名称是必填项。")]
+        [StringLength(50, ErrorMessage = "学校名称不能超过 50 个字符。")]
+        public string SchoolName { get; set; }
+
+        [StringLength(100, ErrorMessage = "地址不能超过 100 个字符。")]
+        public string Address { get; set; }
+    }
+}

Entities/DTO/School/SchoolDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class SchoolDto
+    {
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "学校名称是必填项。")]
+        [StringLength(50, ErrorMessage = "学校名称不能超过 50 个字符。")]
+        public string SchoolName { get; set; }
+
+        [StringLength(100, ErrorMessage = "地址不能超过 100 个字符。")]
+        public string Address { get; set; }
+
+        public DateTime CreateTime { get; set; }
+    }
+}

Entities/DTO/School/SchoolResponseDto.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class SchoolResponseDto
+    {
+        public int SchoolId { get; set; }
+        public string SchoolName { get; set; }
+        public string Address { get; set; }
+        public DateTime CreateTime { get; set; }
+
+        // Navigation properties for response
+        public ICollection<GradeDto> Grades { get; set; }
+    }
+}

Entities/DTO/School/UpdateSchoolDto.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class UpdateSchoolDto
+    {
+        [Required(ErrorMessage = "学校ID是必填项。")]
+        public Guid SchoolId { get; set; }
+
+        [Required(ErrorMessage = "学校名称是必填项。")]
+        [StringLength(50, ErrorMessage = "学校名称不能超过 50 个字符。")]
+        public string SchoolName { get; set; }
+
+        [StringLength(100, ErrorMessage = "地址不能超过 100 个字符。")]
+        public string Address { get; set; }
+    }
+}