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/Infrastructure/Persistence/AppDbContext.cs

@@ -43,6 +43,8 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
         Set<SchedulePublishJob>();
     public DbSet<CourseSelectionRound> CourseSelectionRounds =>
         Set<CourseSelectionRound>();
+    public DbSet<CourseSelectionRoundGrade> CourseSelectionRoundGrades =>
+        Set<CourseSelectionRoundGrade>();
     public DbSet<CourseSelectionOffering> CourseSelectionOfferings =>
         Set<CourseSelectionOffering>();
     public DbSet<CourseEnrollment> CourseEnrollments => Set<CourseEnrollment>();
@@ -477,6 +479,17 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
                 .OnDelete(DeleteBehavior.Restrict);
         });
 
+        builder.Entity<CourseSelectionRoundGrade>(entity =>
+        {
+            entity.HasIndex(x => new { x.CourseSelectionRoundId, x.Grade })
+                .IsUnique();
+            entity.HasIndex(x => x.Grade);
+            entity.HasOne(x => x.CourseSelectionRound)
+                .WithMany(x => x.EligibleGrades)
+                .HasForeignKey(x => x.CourseSelectionRoundId)
+                .OnDelete(DeleteBehavior.Cascade);
+        });
+
         builder.Entity<CourseSelectionOffering>(entity =>
         {
             entity.Property(x => x.Notes).HasMaxLength(500);