1. 教室容量按 1/2 计算

- ExamArrangementService.cs: SelectRooms() 使用 Capacity / 2 选择房间、计算剩余座位和总容量
  - MakeupExamArrangementService.cs: 数据库查询用 x.Capacity >= enrolledCount * 2（等价于 Capacity/2 >=
  enrolledCount），消息显示有效座位数

  2. 教学楼限制多选

  - Domain: ExamSession 和 MakeupExamSession 新增 RequiredBuildingIds (JSON string)，保留旧 RequiredBuildingId 向后兼容
  - Service: RoomGroupKey 改为字符串键确保值相等；GroupKey() 合并新旧字段
  - Controller: 请求 DTO 增加 RequiredBuildingIds (Guid 数组)，响应包含该字段
  - DB: MySQL 迁移 + SQLite migrator 添加新列
  - Frontend: <el-select> 改为 multiple，新增 parseBuildingIds() 解析服务器返回的 JSON

  3. 导出签名单后台任务

  - 新增: ExamSignInExportJob 实体、ExamSignInExportJobProcessor、ExamSignInExportJobStatus 枚举
  - BackgroundJobKind: 新增 ExamSignInExport = 5
  - RabbitMQ: routing key exam.sign-in-export，队列 jiaowu.background-jobs.exam.sign-in-export
  - API: POST /sign-in-export 创建任务返回 202；GET /sign-in-exports/{jobId} 查询状态；GET
  /sign-in-exports/{jobId}/download 下载文件
  - Frontend: 导出改为异步任务 + 轮询 + 自动下载，显示进度条
  - 恢复: OutboxPublisher 启动时恢复未完成的任务，重试超限自动标记失败

src/Jiaowu.Api/Controllers/ExamsController.cs

@@ -205,6 +205,7 @@ public sealed class ExamsController(
                 item.StartsAt,
                 item.EndsAt,
                 item.RequiredBuildingId,
+                item.RequiredBuildingIds,
                 RequiredBuildingName = item.RequiredBuilding != null
                     ? item.RequiredBuilding.Name
                     : null,
@@ -323,6 +324,7 @@ public sealed class ExamsController(
                 item.StartsAt,
                 item.EndsAt,
                 item.RequiredBuildingId,
+                item.RequiredBuildingIds,
                 item.RequiredBuildingName,
                 item.RequiredInvigilatorCount,
                 item.Notes,
@@ -374,6 +376,7 @@ public sealed class ExamsController(
             StartsAt = startsAt,
             EndsAt = endsAt,
             RequiredBuildingId = request.RequiredBuildingId,
+            RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds),
             RequiredInvigilatorCount = request.RequiredInvigilatorCount,
             Notes = Normalize(request.Notes),
             Invigilators = (request.InvigilatorIds ?? []).Distinct().Select(id =>
@@ -453,6 +456,7 @@ public sealed class ExamsController(
             StartsAt = startsAt,
             EndsAt = endsAt,
             RequiredBuildingId = request.RequiredBuildingId,
+            RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds),
             RequiredInvigilatorCount = request.RequiredInvigilatorCount,
             Notes = Normalize(request.Notes)
         }));
@@ -505,6 +509,7 @@ public sealed class ExamsController(
         session.StartsAt = startsAt;
         session.EndsAt = endsAt;
         session.RequiredBuildingId = request.RequiredBuildingId;
+        session.RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds);
         session.RequiredInvigilatorCount = request.RequiredInvigilatorCount;
         session.Notes = Normalize(request.Notes);
 
@@ -1123,6 +1128,89 @@ public sealed class ExamsController(
             $"考场签名单-{FileNamePart(plan.Name)}.xlsx");
     }
 
+    // ═══════════════════════════════════════════
+    //  Sign-in sheet export (background job)
+    // ═══════════════════════════════════════════
+
+    [HttpPost("plans/{id:guid}/sign-in-export")]
+    [Authorize(Roles = Managers)]
+    public async Task<ActionResult> StartSignInExport(
+        Guid id,
+        CancellationToken cancellationToken)
+    {
+        var planExists = await db.ExamPlans.AsNoTracking()
+            .AnyAsync(x => x.Id == id, cancellationToken);
+        if (!planExists) return NotFound();
+
+        var userId = currentUserDataScope.Current.UserId;
+        var job = new ExamSignInExportJob
+        {
+            PlanId = id,
+            RequestedByUserId = userId == Guid.Empty ? null : userId,
+            CurrentStep = "等待后台生成"
+        };
+        db.ExamSignInExportJobs.Add(job);
+        db.BackgroundJobOutboxMessages.Add(
+            BackgroundJobOutboxMessage.Create(
+                BackgroundJobKind.ExamSignInExport,
+                job.Id));
+        await db.SaveChangesAsync(cancellationToken);
+
+        return AcceptedAtAction(
+            nameof(GetSignInExportJob),
+            new { jobId = job.Id },
+            new
+            {
+                jobId = job.Id,
+                status = job.Status.ToString(),
+                message = "考场签名单导出任务已提交。"
+            });
+    }
+
+    [HttpGet("sign-in-exports/{jobId:guid}")]
+    [Authorize(Roles = Managers)]
+    public async Task<ActionResult> GetSignInExportJob(
+        Guid jobId,
+        CancellationToken cancellationToken)
+    {
+        var job = await db.ExamSignInExportJobs.AsNoTracking()
+            .FirstOrDefaultAsync(x => x.Id == jobId, cancellationToken);
+        if (job is null) return NotFound();
+        return Ok(new
+        {
+            job.Id,
+            job.PlanId,
+            Status = job.Status.ToString(),
+            job.FileName,
+            job.FileSize,
+            job.CurrentStep,
+            job.ErrorMessage,
+            job.CreatedAt,
+            job.StartedAt,
+            job.CompletedAt
+        });
+    }
+
+    [HttpGet("sign-in-exports/{jobId:guid}/download")]
+    [Authorize(Roles = Managers)]
+    public async Task<ActionResult> DownloadSignInExport(
+        Guid jobId,
+        CancellationToken cancellationToken)
+    {
+        var job = await db.ExamSignInExportJobs.AsNoTracking()
+            .FirstOrDefaultAsync(
+                x => x.Id == jobId &&
+                     x.Status == ExamSignInExportJobStatus.Succeeded,
+                cancellationToken);
+        if (job is null || job.FileBytes is null || job.FileBytes.Length == 0)
+            return NotFound();
+
+        return File(
+            job.FileBytes,
+            ExcelWorkbookHelper.ContentType,
+            job.FileName ?? "考场签名单.xlsx");
+    }
+
     // ═══════════════════════════════════════════
     //  My schedule
     // ═══════════════════════════════════════════
@@ -1573,6 +1661,10 @@ public sealed class ExamsController(
     });
     private static string? Normalize(string? value) =>
         string.IsNullOrWhiteSpace(value) ? null : value.Trim();
+    private static string? SerializeBuildingIds(IReadOnlyCollection<Guid>? ids) =>
+        ids is { Count: > 0 }
+            ? System.Text.Json.JsonSerializer.Serialize(ids)
+            : null;
     private static string FileNamePart(string value)
     {
         var invalidCharacters = Path.GetInvalidFileNameChars().ToHashSet();
@@ -1601,6 +1693,7 @@ public sealed record CreateExamSessionRequest(
     [Range(1, 30)] int StartPeriod,
     [Range(1, 6)] int PeriodCount,
     Guid? RequiredBuildingId,
+    IReadOnlyCollection<Guid>? RequiredBuildingIds,
     [Range(1, 10)] int RequiredInvigilatorCount,
     IReadOnlyCollection<Guid>? InvigilatorIds,
     [MaxLength(500)] string? Notes);
@@ -1611,6 +1704,7 @@ public sealed record CreateExamSessionsBatchRequest(
     [Range(1, 30)] int StartPeriod,
     [Range(1, 6)] int PeriodCount,
     Guid? RequiredBuildingId,
+    IReadOnlyCollection<Guid>? RequiredBuildingIds,
     [Range(1, 10)] int RequiredInvigilatorCount,
     [MaxLength(500)] string? Notes);