Deliveries run on a Mongo job queue with leases, backoff and per-host limits

The single serial DeliveryWorker is replaced by Infrastructure/Jobs:
- Job rows are leased with one FindOneAndUpdate (oldest RunAt first, a
  two-minute lease) and a reaper returns expired leases every 30 s;
- enqueueing wakes the workers, which otherwise poll every five seconds;
- delivery runs eight at a time with at most two per host, so a slow or
  dead server holds two slots, not the queue;
- a failure waits n^4 + 15 + jitter seconds (Mastodon's curve) for up to
  16 attempts; a 4xx other than 408/429 is final, a 429 honours
  Retry-After;
- RemoteInstance is a per-host circuit breaker: ten consecutive failures
  quarantine a host for an hour, doubling to a week, and its jobs wait
  without spending attempts;
- a delivery is queued once per activity and inbox (unique DedupeKey), and
  finished jobs expire after seven days (TTL on FinishedAt).

Migration _004 moves pending Delivery rows into jobs and marks them
abandoned, so a rollback to the old worker cannot send them twice.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Middleware/SocialPubConfigurations.cs

@@ -15,6 +15,7 @@ using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Outbox;
 using PrivaPub.Federation.Inbox;
 using PrivaPub.Infrastructure.Http;
+using PrivaPub.Infrastructure.Jobs;
 using Microsoft.Extensions.Options;
 
 namespace PrivaPub.Middleware
@@ -51,7 +52,10 @@ namespace PrivaPub.Middleware
 				.AddSingleton<IRemoteActorService, RemoteActorService>()
 				.AddSingleton<IDeliveryService, DeliveryService>()
 				.AddTransient<IInboxService, InboxService>()
-				.AddHostedService<DeliveryWorker>();
+				.AddSingleton<IJobQueue, JobQueue>()
+				.AddSingleton<IHostCircuitBreaker, HostCircuitBreaker>()
+				.AddSingleton<IJobHandler, DeliveryJobHandler>()
+				.AddHostedService<JobWorker>();
 		}
 		public static IServiceCollection PrivaPubAuthServicesConfiguration(this IServiceCollection service, IConfiguration configuration)
 		{