第二阶段优化已完成，重点是提高后台任务吞吐并降低 Outbox 数据库开销。

RabbitMQ 默认从单一回调通道提升为三类任务独立并行，跨类型并发能力由 1 提升到 3；每类还能独立配置 1–16 个消费者。[RabbitMqBackgroundJobs.cs (line 143)](E:/jiaowu/src/Jiaowu.Api/Infrastructure/BackgroundJobs/RabbitMqBackgroundJobs.cs:143)
InMemory 开发模式同步改为按任务类型隔离队列，避免某类长任务堵塞其他任务。
租约恢复检查从“每发布一条执行一次”改为默认每 60 秒维护一次。
启动恢复改为数据库 NOT EXISTS 查询，不再把全部历史 Outbox 加载进内存。[BackgroundJobOutboxPublisher.cs (line 9)](E:/jiaowu/src/Jiaowu.Api/Infrastructure/BackgroundJobs/BackgroundJobOutboxPublisher.cs:9)
完成消息默认保留 14 天，之后按每批 500 条清理，并增加对应组合索引。
/health/messaging 现在返回各状态积压量、过期租约和最老任务等待时间。[BackgroundJobMonitoringService.cs (line 16)](E:/jiaowu/src/Jiaowu.Api/Infrastructure/BackgroundJobs/BackgroundJobMonitoringService.cs:16)
新增 Jiaowu.BackgroundJobs 运行时指标，覆盖发布量、处理量、发布耗时、处理耗时和清理量。[BackgroundJobTelemetry.cs (line 9)](E:/jiaowu/src/Jiaowu.Api/Infrastructure/BackgroundJobs/BackgroundJobTelemetry.cs:9)
配置、Compose 和调优建议已更新。[README.md (line 257)](E:/jiaowu/README.md:257)

src/Jiaowu.Api/Infrastructure/BackgroundJobs/BackgroundJobTransport.cs

@@ -21,26 +21,31 @@ public interface IBackgroundJobTransport
 
 public sealed class InMemoryBackgroundJobTransport : IBackgroundJobTransport
 {
-    private readonly Channel<BackgroundJobEnvelope> _channel =
-        Channel.CreateBounded<BackgroundJobEnvelope>(
-            new BoundedChannelOptions(256)
-            {
-                FullMode = BoundedChannelFullMode.Wait,
-                SingleReader = true,
-                SingleWriter = false
-            });
+    private readonly IReadOnlyDictionary<
+        BackgroundJobKind,
+        Channel<BackgroundJobEnvelope>> _channels =
+        Enum.GetValues<BackgroundJobKind>().ToDictionary(
+            kind => kind,
+            _ => Channel.CreateBounded<BackgroundJobEnvelope>(
+                new BoundedChannelOptions(256)
+                {
+                    FullMode = BoundedChannelFullMode.Wait,
+                    SingleReader = false,
+                    SingleWriter = false
+                }));
 
     public bool IsDurable => false;
 
     public ValueTask PublishAsync(
         BackgroundJobEnvelope message,
         CancellationToken cancellationToken) =>
-        _channel.Writer.WriteAsync(message, cancellationToken);
+        _channels[message.JobKind].Writer.WriteAsync(message, cancellationToken);
 
     public Task<bool> CheckHealthAsync(CancellationToken cancellationToken) =>
         Task.FromResult(true);
 
     public IAsyncEnumerable<BackgroundJobEnvelope> ReadAllAsync(
+        BackgroundJobKind kind,
         CancellationToken cancellationToken) =>
-        _channel.Reader.ReadAllAsync(cancellationToken);
+        _channels[kind].Reader.ReadAllAsync(cancellationToken);
 }