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/MakeupExamsController.cs

@@ -110,6 +110,7 @@ public sealed class MakeupExamsController(
                         item.StartsAt,
                         item.EndsAt,
                         item.RequiredBuildingId,
+                        item.RequiredBuildingIds,
                         RequiredBuildingName = item.RequiredBuilding != null
                         ? item.RequiredBuilding.Name : null,
                         item.RequiredInvigilatorCount,
@@ -220,6 +221,7 @@ public sealed class MakeupExamsController(
             StartsAt = startsAt,
             EndsAt = endsAt,
             RequiredBuildingId = request.RequiredBuildingId,
+            RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds),
             RequiredInvigilatorCount = request.RequiredInvigilatorCount,
             Notes = Normalize(request.Notes),
             Invigilators = (request.InvigilatorIds ?? []).Distinct().Select(id =>
@@ -288,6 +290,7 @@ public sealed class MakeupExamsController(
             StartsAt = startsAt,
             EndsAt = endsAt,
             RequiredBuildingId = request.RequiredBuildingId,
+            RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds),
             RequiredInvigilatorCount = request.RequiredInvigilatorCount,
             Notes = Normalize(request.Notes)
         }));
@@ -339,6 +342,7 @@ public sealed class MakeupExamsController(
         session.StartsAt = startsAt;
         session.EndsAt = endsAt;
         session.RequiredBuildingId = request.RequiredBuildingId;
+        session.RequiredBuildingIds = SerializeBuildingIds(request.RequiredBuildingIds);
         session.RequiredInvigilatorCount = request.RequiredInvigilatorCount;
         session.Notes = Normalize(request.Notes);
 
@@ -1222,6 +1226,10 @@ public sealed class MakeupExamsController(
     });
     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;
 }
 
 // ═══════════════════════════════════════════
@@ -1240,6 +1248,7 @@ public sealed record CreateMakeupExamSessionRequest(
     [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);
@@ -1250,6 +1259,7 @@ public sealed record CreateMakeupExamSessionsBatchRequest(
     [Range(1, 30)] int StartPeriod,
     [Range(1, 6)] int PeriodCount,
     Guid? RequiredBuildingId,
+    IReadOnlyCollection<Guid>? RequiredBuildingIds,
     [Range(1, 10)] int RequiredInvigilatorCount,
     [MaxLength(500)] string? Notes);