已把选课的“年级划分”和限制规则补成完整闭环：

选课批次可指定多个适用年级；留空兼容原有“全部年级”。
新增可选的“最多课程门数”，与原有学分上限并行。
年级、门数限制统一作用于学生可见批次、自主选课、候补、管理员代选和自动递补。
普通候选名单遵守年级及行政班范围；强制选课可绕过业务限制，但仍严格遵守管理员学院权限。
管理页面会展示“2026 级 · 最多 6 门 · 最多 30 学分”，名单和候补列表也增加年级列。
新增 MySQL 迁移：[CourseSelectionGradeLimits.cs](E:/jiaowu/src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260726180000_CourseSelectionGradeLimits.cs)。

src/Jiaowu.Api/Domain/Academic/CourseSelectionEntities.cs

@@ -11,12 +11,21 @@ public sealed class CourseSelectionRound : EntityBase
     public DateTime EndsAt { get; set; }
     public DateTime WithdrawalEndsAt { get; set; }
     public decimal MaxCredits { get; set; } = 30;
+    public int? MaxCourseCount { get; set; }
     public CourseSelectionRoundStatus Status { get; set; } =
         CourseSelectionRoundStatus.Draft;
     public string? Notes { get; set; }
+    public ICollection<CourseSelectionRoundGrade> EligibleGrades { get; set; } = [];
     public ICollection<CourseSelectionOffering> Offerings { get; set; } = [];
 }
 
+public sealed class CourseSelectionRoundGrade : EntityBase
+{
+    public Guid CourseSelectionRoundId { get; set; }
+    public CourseSelectionRound? CourseSelectionRound { get; set; }
+    public int Grade { get; set; }
+}
+
 public sealed class CourseSelectionOffering : EntityBase
 {
     public Guid CourseSelectionRoundId { get; set; }