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/BackgroundJobRunner.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using Jiaowu.Api.Domain.Academic;
 using Jiaowu.Api.Domain.System;
 using Jiaowu.Api.Infrastructure.Exams;
@@ -28,12 +29,14 @@ public sealed class BackgroundJobRunner(
     IServiceScopeFactory scopeFactory,
     IBackgroundJobTransport transport,
     BackgroundJobOptions options,
+    BackgroundJobTelemetry telemetry,
     ILogger<BackgroundJobRunner> logger)
 {
     public async Task<BackgroundJobRunResult> RunAsync(
         BackgroundJobEnvelope message,
         CancellationToken cancellationToken)
     {
+        var startedAt = Stopwatch.GetTimestamp();
         var token = Guid.NewGuid();
         var claim = await TryClaimAsync(message, token, cancellationToken);
         if (!claim.Claimed)
@@ -48,6 +51,10 @@ public sealed class BackgroundJobRunner(
             await MarkJobRetryLimitExceededAsync(message, cancellationToken);
             await MarkCompletedAsync(message.OutboxMessageId, token,
                 cancellationToken);
+            telemetry.RecordProcessing(
+                message.JobKind,
+                "retry-limit",
+                Stopwatch.GetElapsedTime(startedAt));
             return BackgroundJobRunResult.Completed;
         }
 
@@ -85,6 +92,10 @@ public sealed class BackgroundJobRunner(
 
             await MarkCompletedAsync(message.OutboxMessageId, token,
                 cancellationToken);
+            telemetry.RecordProcessing(
+                message.JobKind,
+                "completed",
+                Stopwatch.GetElapsedTime(startedAt));
             return BackgroundJobRunResult.Completed;
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
@@ -103,6 +114,10 @@ public sealed class BackgroundJobRunner(
                 token,
                 exception,
                 CancellationToken.None);
+            telemetry.RecordProcessing(
+                message.JobKind,
+                "retry",
+                Stopwatch.GetElapsedTime(startedAt));
             return BackgroundJobRunResult.Retry(TimeSpan.FromSeconds(2));
         }
         finally