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/Logs/ConsumeUploadIntentsLog.cs

@@ -0,0 +1,5 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record ConsumeUploadIntentsLog(
+        Guid[] IntentIds);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/CopyFileInBucketLog.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record CopyFileInBucketLog(
+        string DestinationBucket,
+        string DestinationKey);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/DownloadFileFromBucketLog.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record DownloadFileFromBucketLog(
+        string Bucket,
+        string Key,
+        string DestinationFilePath);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/DownloadFileFromUrlLog.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record DownloadFileFromUrlLog(
+        string SourceUrl,
+        string DestinationFilePath);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/DownloadYouTubeAudioLog.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record DownloadYouTubeAudioLog(
+        string VideoId,
+        string DestinationFilePath);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/GeneratePictureWorkflowPathsLog.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record GeneratePictureWorkflowPathsLog(string WorkingDirectory);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/ResizePictureLog.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record ResizePictureLog(string DestinationFilePath);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/TranscodeAudioLog.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record TranscodeAudioLog(string WorkingDirectory);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/TransferFilesToBucketLog.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record TransferFilesToBucketLog(string DestinationBucket, string[] UploadedKeys);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/UpdatePlayListPictureLog.cs

@@ -0,0 +1,9 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record UpdatePlayListPictureLog(
+        Guid PlayListId,
+        string PreviousOriginalPictureKey,
+        string PreviousSmallPictureKey,
+        string PreviousMediumPictureKey,
+        string PreviousLargePictureKey);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/UpdateTrackAudioLog.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record UpdateTrackAudioLog(
+        Guid TrackId,
+        string? PreviousAudioFolderName,
+        double PreviousDurationSeconds);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/UpdateTrackPictureLog.cs

@@ -0,0 +1,9 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record UpdateTrackPictureLog(
+        Guid TrackId,
+        string? PreviousOriginalPictureKey,
+        string? PreviousSmallPictureKey,
+        string? PreviousMediumPictureKey,
+        string? PreviousLargePictureKey);
+}

backend/Core/Musify.Infrastructure/MassTransit/Logs/UploadFileToBucketLog.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record UploadFileToBucketLog(string DestinationBucket, string DestinationKey);
+}