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/CourseSelection/CourseSelectionRules.cs

@@ -14,6 +14,10 @@ public static class CourseSelectionRules
         round.Status == CourseSelectionRoundStatus.Open &&
         nowUtc <= round.WithdrawalEndsAt;
 
+    public static bool CanPromoteWaitlist(CourseSelectionRound round, DateTime nowUtc) =>
+        round.Status == CourseSelectionRoundStatus.Open &&
+        nowUtc <= round.WithdrawalEndsAt;
+
     public static bool SupportsProxyEnrollment(CourseNature nature) =>
         nature == CourseNature.GeneralRequired;
 
@@ -68,4 +72,7 @@ public static class CourseSelectionRules
     /// <summary>Retake expanded capacity: ceiling(original * 1.15).</summary>
     public static int RetakeCapacity(int originalCapacity) =>
         (int)Math.Ceiling(originalCapacity * 1.15);
+
+    public static int EffectiveCapacity(int originalCapacity, bool isRetake) =>
+        isRetake ? RetakeCapacity(originalCapacity) : originalCapacity;
 }

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContext.cs

@@ -500,6 +500,12 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
             entity.HasIndex(x => new { x.CourseSelectionOfferingId, x.StudentId })
                 .IsUnique();
             entity.HasIndex(x => new { x.StudentId, x.Status });
+            entity.HasIndex(x => new
+            {
+                x.CourseSelectionOfferingId,
+                x.Status,
+                x.WaitlistedAt
+            });
             entity.HasOne(x => x.CourseSelectionOffering)
                 .WithMany(x => x.Enrollments)
                 .HasForeignKey(x => x.CourseSelectionOfferingId)

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 =
     [
         """

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");