已完成“选课候补与自动递补”完整流程。

学生端：满员后可加入/取消候补，展示候补顺位和当前候补人数。
自动递补：退课释放名额后，按候补时间顺序重新校验学籍、范围、学分及时间冲突；不合格者自动失效并继续下一位。
管理端：教学班显示候补人数，名单抽屉提供候补队列及移出操作。
状态通知：递补成功、资格失效、批次关闭、管理员移出均会通知学生。
并发安全：退课和递补放在 Serializable 事务中执行。
数据库：已补充 SQLite 开发迁移和 MySQL 正式迁移。

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260726170000_CourseSelectionWaitlist.cs

@@ -0,0 +1,37 @@
+using Jiaowu.Api.Infrastructure.Persistence;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql;
+
+[DbContext(typeof(AppDbContext))]
+[Migration("20260726170000_CourseSelectionWaitlist")]
+public partial class CourseSelectionWaitlist : Migration
+{
+    protected override void Up(MigrationBuilder migrationBuilder)
+    {
+        migrationBuilder.AddColumn<DateTime>(
+            name: "WaitlistedAt",
+            table: "CourseEnrollments",
+            type: "datetime(6)",
+            nullable: true);
+
+        migrationBuilder.CreateIndex(
+            name: "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt",
+            table: "CourseEnrollments",
+            columns: new[] { "CourseSelectionOfferingId", "Status", "WaitlistedAt" });
+    }
+
+    protected override void Down(MigrationBuilder migrationBuilder)
+    {
+        migrationBuilder.DropIndex(
+            name: "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt",
+            table: "CourseEnrollments");
+
+        migrationBuilder.DropColumn(
+            name: "WaitlistedAt",
+            table: "CourseEnrollments");
+    }
+}

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/AppDbContextModelSnapshot.cs

@@ -732,6 +732,9 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
                     b.Property<DateTime>("UpdatedAt")
                         .HasColumnType("datetime(6)");
 
+                    b.Property<DateTime?>("WaitlistedAt")
+                        .HasColumnType("datetime(6)");
+
                     b.Property<DateTime?>("WithdrawnAt")
                         .HasColumnType("datetime(6)");
 
@@ -740,6 +743,8 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
                     b.HasIndex("CourseSelectionOfferingId", "StudentId")
                         .IsUnique();
 
+                    b.HasIndex("CourseSelectionOfferingId", "Status", "WaitlistedAt");
+
                     b.HasIndex("StudentId", "Status");
 
                     b.ToTable("CourseEnrollments");