T1: tests stop sharing state they don't own

- JobQueue takes an optional scope, so a test's worker leases and reaps only its own jobs.
- The dead-host delivery test runs alone (Exclusive), on its own jobs, and cleans up the
  breaker rows it trips; the breaker has tests of its own on unique hosts.
- Index and migration tests run alone: they drop indexes and rewrite every post.
- DomainBlocks.Load replaces reflection and a database-wide block in tests.
- Harness.Outgoing sees only deliveries queued since the harness started: Peer ports are
  reused within a run, which made the circle test flaky.
- Two pure-logic tests leave Mongo-gated classes, so CI runs them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub.Tests/Infrastructure/HostCircuitBreakerTests.cs

@@ -0,0 +1,67 @@
+using Microsoft.Extensions.Caching.Memory;
+
+using MongoDB.Entities;
+
+using PrivaPub.Infrastructure.Jobs;
+using PrivaPub.Models.Jobs;
+using PrivaPub.Tests.Support;
+
+namespace PrivaPub.Tests.Infrastructure
+{
+	[Trait("Category", "Integration")]
+	public sealed class HostCircuitBreakerTests : IAsyncLifetime
+	{
+		readonly string _host = $"breaker{Guid.NewGuid():N}.example";
+
+		public ValueTask InitializeAsync()
+		{
+			Assert.SkipUnless(MongoFixture.Enabled, MongoFixture.Skip);
+			return ValueTask.CompletedTask;
+		}
+
+		public async ValueTask DisposeAsync()
+		{
+			if (MongoFixture.Enabled)
+				await DB.Default.DeleteAsync<RemoteInstance>(i => i.Host == _host);
+		}
+
+		[Fact]
+		public async Task A_host_is_quarantined_only_past_the_threshold_and_freed_by_a_success()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var breaker = new HostCircuitBreaker(new MemoryCache(new MemoryCacheOptions()));
+
+			for (var i = 1; i < HostCircuitBreaker.Threshold; i++)
+				await breaker.Failed(_host, "503", token);
+			Assert.Null(await breaker.UnavailableUntil(_host, token));
+
+			await breaker.Failed(_host, "503", token);
+			var until = await breaker.UnavailableUntil(_host, token);
+			Assert.NotNull(until);
+			Assert.InRange(until.Value, DateTime.UtcNow.AddMinutes(55), DateTime.UtcNow.AddMinutes(65));
+			var instance = await DB.Default.Find<RemoteInstance>().Match(i => i.Host == _host).ExecuteFirstAsync(token);
+			Assert.Equal(HostCircuitBreaker.Threshold, instance.ConsecutiveFailures);
+			Assert.Equal("503", instance.LastError);
+
+			await breaker.Succeeded(_host, token);
+			Assert.Null(await breaker.UnavailableUntil(_host, token));
+			instance = await DB.Default.Find<RemoteInstance>().Match(i => i.Host == _host).ExecuteFirstAsync(token);
+			Assert.Equal(0, instance.ConsecutiveFailures);
+			Assert.NotNull(instance.LastSuccessAt);
+		}
+
+		[Fact]
+		public async Task A_healthy_host_is_written_at_most_once_an_hour()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var breaker = new HostCircuitBreaker(new MemoryCache(new MemoryCacheOptions()));
+
+			await breaker.Succeeded(_host, token);
+			var first = (await DB.Default.Find<RemoteInstance>().Match(i => i.Host == _host).ExecuteFirstAsync(token)).LastSuccessAt;
+			await breaker.Succeeded(_host, token);
+			var second = (await DB.Default.Find<RemoteInstance>().Match(i => i.Host == _host).ExecuteFirstAsync(token)).LastSuccessAt;
+
+			Assert.Equal(first, second);
+		}
+	}
+}