feat: ship agent-first mission control v0.2.57

backend/Services/IDomainEventStreamService.cs

@@ -0,0 +1,31 @@
+using System.Threading.Channels;
+using Nexus.Api.Models;
+
+namespace Nexus.Api.Services;
+
+public interface IDomainEventStreamService
+{
+    long CurrentSequence { get; }
+
+    DomainEventSubscription Subscribe(IReadOnlySet<string> channels);
+}
+
+public sealed class DomainEventSubscription(
+    ChannelReader<DomainEventDto> reader,
+    long startingSequence,
+    Func<ValueTask> disposeAsync) : IAsyncDisposable
+{
+    public ChannelReader<DomainEventDto> Reader { get; } = reader;
+    public long StartingSequence { get; } = startingSequence;
+
+    public ValueTask DisposeAsync() => disposeAsync();
+}
+
+public sealed class DomainEventSubscriberOverflowException :
+    InvalidOperationException
+{
+    public DomainEventSubscriberOverflowException()
+        : base("The domain-event subscriber queue overflowed and requires a REST resync.")
+    {
+    }
+}