Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

backend/Core/Musify.Infrastructure/MassTransit/Consumers/DownloadYouTubeTrackConsumer.cs

@@ -0,0 +1,26 @@
+using MassTransit;
+using Musify.Application.Events;
+using Musify.Infrastructure.MassTransit.RoutingSlip.Builders;
+
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    public class DownloadYouTubeTrackConsumer(
+        IBus bus,
+        YouTubeTrackRoutingSlipBuilder routingSlipBuilder)
+        : IConsumer<DownloadYouTubeTrackEvent>
+    {
+        public async Task Consume(ConsumeContext<DownloadYouTubeTrackEvent> consumeContext)
+        {
+            var audioRoutingSlip = routingSlipBuilder
+                .BuildAudioWorkflow(consumeContext.Message, consumeContext.CorrelationId)
+                .Build();
+
+            var pictureRoutingSlip = routingSlipBuilder
+                .BuildPictureWorkflow(consumeContext.Message, consumeContext.CorrelationId)
+                .Build();
+
+            await bus.Execute(audioRoutingSlip);
+            await bus.Execute(pictureRoutingSlip);
+        }
+    }
+}