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.Tests/Federation/InboxScenarioTests.cs

@@ -6,6 +6,7 @@ using MongoDB.Entities;
 using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Inbox;
 using PrivaPub.Federation.Outbox;
+using PrivaPub.Infrastructure.Jobs;
 using PrivaPub.Models;
 using PrivaPub.Models.Group;
 using PrivaPub.Models.Post;
@@ -36,7 +37,7 @@ namespace PrivaPub.Tests.Federation
 			var cache = new MemoryCache(new MemoryCacheOptions());
 			_local = new LocalActorService(new DbEntities(), new StaticOptions<AppConfiguration>(new AppConfiguration { BackendBaseAddress = Base }));
 			var remote = new RemoteActorService(Peer.Http(cache), _local, cache, new DbEntities());
-			_inbox = new InboxService(new DbEntities(), _local, remote, new DeliveryService(new DbEntities()), NullLogger<InboxService>.Instance);
+			_inbox = new InboxService(new DbEntities(), _local, remote, new DeliveryService(new DbEntities(), new JobQueue()), NullLogger<InboxService>.Instance);
 		}
 
 		public async ValueTask DisposeAsync()