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/BackgroundJobOutboxPublisher.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using Jiaowu.Api.Domain.Academic;
 using Jiaowu.Api.Domain.System;
 using Jiaowu.Api.Infrastructure.Persistence;
@@ -9,17 +10,26 @@ public sealed class BackgroundJobOutboxPublisher(
     IServiceScopeFactory scopeFactory,
     IBackgroundJobTransport transport,
     BackgroundJobOptions options,
+    BackgroundJobTelemetry telemetry,
     ILogger<BackgroundJobOutboxPublisher> logger) : BackgroundService
 {
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
         await PrepareForStartupAsync(stoppingToken);
+        var nextMaintenanceAt = DateTime.MinValue;
 
         while (!stoppingToken.IsCancellationRequested)
         {
             try
             {
-                await RecoverExpiredProcessingAsync(stoppingToken);
+                if (DateTime.UtcNow >= nextMaintenanceAt)
+                {
+                    await RecoverExpiredProcessingAsync(stoppingToken);
+                    await CleanupCompletedAsync(stoppingToken);
+                    nextMaintenanceAt = DateTime.UtcNow.AddSeconds(
+                        options.MaintenanceIntervalSeconds);
+                }
+
                 var claimed = await ClaimNextAsync(stoppingToken);
                 if (claimed is null)
                 {
@@ -62,51 +72,107 @@ public sealed class BackgroundJobOutboxPublisher(
         await AddMissingOutboxMessagesAsync(db, cancellationToken);
     }
 
-    private static async Task AddMissingOutboxMessagesAsync(
+    private async Task AddMissingOutboxMessagesAsync(
         AppDbContext db,
         CancellationToken cancellationToken)
     {
-        var existing = await db.BackgroundJobOutboxMessages.AsNoTracking()
-            .Select(x => new { x.JobKind, x.JobId })
-            .ToListAsync(cancellationToken);
-        var existingKeys = existing
-            .Select(x => (x.JobKind, x.JobId))
-            .ToHashSet();
-
         var automaticJobs = await db.AutomaticScheduleJobs.AsNoTracking()
-            .Where(x => x.Status == AutomaticScheduleJobStatus.Queued ||
-                        x.Status == AutomaticScheduleJobStatus.Running)
+            .Where(x =>
+                (x.Status == AutomaticScheduleJobStatus.Queued ||
+                 x.Status == AutomaticScheduleJobStatus.Running) &&
+                !db.BackgroundJobOutboxMessages.Any(message =>
+                    message.JobKind == BackgroundJobKind.AutomaticSchedule &&
+                    message.JobId == x.Id))
             .Select(x => x.Id)
             .ToListAsync(cancellationToken);
         var publishJobs = await db.SchedulePublishJobs.AsNoTracking()
-            .Where(x => x.Status == SchedulePublishJobStatus.Queued ||
-                        x.Status == SchedulePublishJobStatus.Running)
+            .Where(x =>
+                (x.Status == SchedulePublishJobStatus.Queued ||
+                 x.Status == SchedulePublishJobStatus.Running) &&
+                !db.BackgroundJobOutboxMessages.Any(message =>
+                    message.JobKind == BackgroundJobKind.SchedulePublish &&
+                    message.JobId == x.Id))
             .Select(x => x.Id)
             .ToListAsync(cancellationToken);
         var makeupJobs = await db.MakeupExamAutoJobs.AsNoTracking()
-            .Where(x => x.Status == MakeupExamAutoJobStatus.Queued ||
-                        x.Status == MakeupExamAutoJobStatus.Running)
+            .Where(x =>
+                (x.Status == MakeupExamAutoJobStatus.Queued ||
+                 x.Status == MakeupExamAutoJobStatus.Running) &&
+                !db.BackgroundJobOutboxMessages.Any(message =>
+                    message.JobKind == BackgroundJobKind.MakeupExamAuto &&
+                    message.JobId == x.Id))
             .Select(x => x.Id)
             .ToListAsync(cancellationToken);
 
-        AddMissing(BackgroundJobKind.AutomaticSchedule, automaticJobs);
-        AddMissing(BackgroundJobKind.SchedulePublish, publishJobs);
-        AddMissing(BackgroundJobKind.MakeupExamAuto, makeupJobs);
-
-        if (db.ChangeTracker.HasChanges())
-            await db.SaveChangesAsync(cancellationToken);
-
-        void AddMissing(BackgroundJobKind kind, IEnumerable<Guid> jobIds)
+        var missingKeys = automaticJobs
+            .Select(id => (BackgroundJobKind.AutomaticSchedule, id))
+            .Concat(publishJobs.Select(id =>
+                (BackgroundJobKind.SchedulePublish, id)))
+            .Concat(makeupJobs.Select(id =>
+                (BackgroundJobKind.MakeupExamAuto, id)))
+            .ToList();
+        foreach (var (kind, jobId) in missingKeys)
         {
-            foreach (var jobId in jobIds)
-            {
-                if (existingKeys.Add((kind, jobId)))
-                {
-                    db.BackgroundJobOutboxMessages.Add(
-                        BackgroundJobOutboxMessage.Create(kind, jobId));
-                }
-            }
+            db.BackgroundJobOutboxMessages.Add(
+                BackgroundJobOutboxMessage.Create(kind, jobId));
         }
+
+        if (!db.ChangeTracker.HasChanges())
+            return;
+
+        try
+        {
+            await db.SaveChangesAsync(cancellationToken);
+        }
+        catch (DbUpdateException)
+        {
+            db.ChangeTracker.Clear();
+            foreach (var (kind, jobId) in missingKeys)
+            {
+                var recovered = await db.BackgroundJobOutboxMessages
+                    .AsNoTracking()
+                    .AnyAsync(
+                        message => message.JobKind == kind &&
+                                   message.JobId == jobId,
+                        cancellationToken);
+                if (!recovered)
+                    throw;
+            }
+
+            logger.LogInformation(
+                "Another application instance recovered {Count} missing outbox messages first.",
+                missingKeys.Count);
+        }
+    }
+
+    private async Task CleanupCompletedAsync(CancellationToken cancellationToken)
+    {
+        await using var scope = scopeFactory.CreateAsyncScope();
+        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
+        var cutoff = DateTime.UtcNow.AddDays(-options.CompletedRetentionDays);
+        var completedIds = await db.BackgroundJobOutboxMessages.AsNoTracking()
+            .Where(x =>
+                x.State == BackgroundJobOutboxState.Completed &&
+                x.CompletedAt != null &&
+                x.CompletedAt < cutoff)
+            .OrderBy(x => x.CompletedAt)
+            .Select(x => x.Id)
+            .Take(options.CleanupBatchSize)
+            .ToListAsync(cancellationToken);
+        if (completedIds.Count == 0)
+            return;
+
+        foreach (var id in completedIds)
+        {
+            db.BackgroundJobOutboxMessages.Remove(
+                new BackgroundJobOutboxMessage { Id = id });
+        }
+        await db.SaveChangesAsync(cancellationToken);
+        telemetry.RecordCleaned(completedIds.Count);
+        logger.LogInformation(
+            "Removed {Count} completed background job outbox messages older than {Cutoff}.",
+            completedIds.Count,
+            cutoff);
     }
 
     private async Task RecoverExpiredProcessingAsync(
@@ -175,6 +241,7 @@ public sealed class BackgroundJobOutboxPublisher(
         Guid token,
         CancellationToken cancellationToken)
     {
+        var startedAt = Stopwatch.GetTimestamp();
         try
         {
             await transport.PublishAsync(message, cancellationToken);
@@ -193,6 +260,11 @@ public sealed class BackgroundJobOutboxPublisher(
                         .SetProperty(x => x.ProcessingToken, (Guid?)null)
                         .SetProperty(x => x.LeaseExpiresAt, (DateTime?)null),
                     cancellationToken);
+            telemetry.RecordPublish(
+                message.JobKind,
+                transport.IsDurable,
+                "published",
+                Stopwatch.GetElapsedTime(startedAt));
         }
         catch (Exception exception)
         {
@@ -213,6 +285,11 @@ public sealed class BackgroundJobOutboxPublisher(
                         .SetProperty(x => x.LeaseExpiresAt, (DateTime?)null)
                         .SetProperty(x => x.LastError, messageText),
                     cancellationToken);
+            telemetry.RecordPublish(
+                message.JobKind,
+                transport.IsDurable,
+                "failed",
+                Stopwatch.GetElapsedTime(startedAt));
             throw;
         }
     }