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/Infrastructure/Exams/MakeupExamArrangementService.cs

@@ -99,7 +99,7 @@ public sealed class MakeupExamArrangementService(AppDbContext db)
                     occupiedRooms.Add(new RoomOccupancy(room.Id, session.StartsAt, session.EndsAt));
                     assignedRooms++;
                     messages.Add(
-                        $"\"{session.TeachingTask!.Course!.Name}\"→{room.Name}({room.Capacity}座)");
+                        $"\"{session.TeachingTask!.Course!.Name}\"→{room.Name}({room.Capacity / 2}座)");
                 }
                 else
                 {
@@ -170,10 +170,13 @@ public sealed class MakeupExamArrangementService(AppDbContext db)
         CancellationToken cancellationToken)
     {
         var query = db.Classrooms.AsNoTracking()
-            .Where(x => x.IsEnabled && x.Capacity >= enrolledCount);
+            .Where(x => x.IsEnabled && x.Capacity >= enrolledCount * 2);
 
+        var buildingIds = ParseBuildingIds(session.RequiredBuildingIds);
         if (session.RequiredBuildingId.HasValue)
-            query = query.Where(x => x.BuildingId == session.RequiredBuildingId.Value);
+            buildingIds.Add(session.RequiredBuildingId.Value);
+        if (buildingIds.Count > 0)
+            query = query.Where(x => buildingIds.Contains(x.BuildingId));
 
         var occupiedRoomIds = occupied
             .Where(x => ExamConflictRules.TimeOverlaps(
@@ -234,4 +237,18 @@ public sealed class MakeupExamArrangementService(AppDbContext db)
             .Take(needed)
             .ToList();
     }
+
+    private static HashSet<Guid> ParseBuildingIds(string? json)
+    {
+        if (string.IsNullOrWhiteSpace(json))
+            return [];
+        try
+        {
+            return System.Text.Json.JsonSerializer.Deserialize<HashSet<Guid>>(json) ?? [];
+        }
+        catch
+        {
+            return [];
+        }
+    }
 }