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/DevelopmentSqliteMigrator.cs

@@ -48,6 +48,8 @@ public sealed class DevelopmentSqliteMigrator(
         "20260725_25_academic_warnings";
     private const string AcademicTermArchivingMigration =
         "20260726_27_academic_term_archiving";
+    private const string CourseSelectionWaitlistMigration =
+        "20260726_28_course_selection_waitlist";
 
     public async Task MigrateAsync(CancellationToken cancellationToken = default)
     {
@@ -344,6 +346,19 @@ public sealed class DevelopmentSqliteMigrator(
             AcademicTermArchivingMigration,
             academicTermArchivingExists ? [] : AcademicTermArchivingStatements,
             cancellationToken);
+
+        var courseSelectionWaitlistExists = await db.Database
+            .SqlQueryRaw<int>(
+                """
+                SELECT COUNT(*) AS "Value"
+                FROM pragma_table_info('CourseEnrollments')
+                WHERE name = 'WaitlistedAt'
+                """)
+            .AnyAsync(value => value > 0, cancellationToken);
+        await ApplyMigrationAsync(
+            CourseSelectionWaitlistMigration,
+            courseSelectionWaitlistExists ? [] : CourseSelectionWaitlistStatements,
+            cancellationToken);
     }
 
     private async Task ApplyMigrationAsync(
@@ -1589,6 +1604,18 @@ public sealed class DevelopmentSqliteMigrator(
         """
     ];
 
+    private static readonly string[] CourseSelectionWaitlistStatements =
+    [
+        """
+        ALTER TABLE "CourseEnrollments"
+        ADD COLUMN "WaitlistedAt" TEXT NULL;
+        """,
+        """
+        CREATE INDEX IF NOT EXISTS "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt"
+        ON "CourseEnrollments" ("CourseSelectionOfferingId", "Status", "WaitlistedAt");
+        """
+    ];
+
     private static readonly string[] CourseAdjustmentsStatements =
     [
         """