chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Infrastructure/MassTransit/Arguments/ConsumeUploadIntentsArguments.cs

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

backend/Musify.Infrastructure/MassTransit/Arguments/CopyFileInBucketArguments.cs

@@ -0,0 +1,8 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record CopyFileInBucketArguments(
+        string SourceBucket,
+        string SourceKey,
+        string DestinationBucket,
+        string DestinationKey);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/DeletePlayListFromDbArguments.cs

@@ -0,0 +1,5 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    internal record DeletePlayListFromDbArguments(
+        Guid PlayListId);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/DeleteTrackFromDbArguments.cs

@@ -0,0 +1,5 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    internal record DeleteTrackFromDbArguments(
+        Guid TrackId);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/DownloadFileFromBucketArguments.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record DownloadFileFromBucketArguments(
+        string Bucket,
+        string Key,
+        string DestinationFilePathVariable);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/GenerateAudioWorkflowPathsArguments.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record GenerateAudioWorkflowPathsArguments(
+        Guid TrackId,
+        string TemporaryRootDirectory,
+        string SourceKey);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/GeneratePictureWorkflowPathsArguments.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record GeneratePictureWorkflowPathsArguments(
+        string TemporaryRootDirectory,
+        string SourceKey);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/MarkPlayListAsRemovingArguments.cs

@@ -0,0 +1,5 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    internal record MarkPlayListAsRemovingArguments(
+        Guid PlayListId);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/MarkTrackAsRemovingArguments.cs

@@ -0,0 +1,5 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    internal record MarkTrackAsRemovingArguments(
+        Guid TrackId);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/PublishPlayListPictureProcessingEventArguments.cs

@@ -0,0 +1,12 @@
+using Musify.Application.Shared;
+
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record PublishPlayListPictureProcessingEventArguments(
+        Guid PlayListId,
+        string Bucket,
+        string FinalPictureKey,
+        ImageSize Small,
+        ImageSize Medium,
+        ImageSize Large);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/PublishTrackProcessingEventsArguments.cs

@@ -0,0 +1,14 @@
+using Musify.Application.Shared;
+
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record PublishTrackProcessingEventsArguments(
+        Guid TrackId,
+        string Bucket,
+        string FinalPictureKey,
+        string FinalAudioKey,
+        string AudioDestinationFolderKey,
+        ImageSize Small,
+        ImageSize Medium,
+        ImageSize Large);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/RemoveFileFromBucketArguments.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record RemoveFileFromBucketArguments(
+        string Bucket,
+        string Key);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/ResizePictureLocalArguments.cs

@@ -0,0 +1,8 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record ResizePictureLocalArguments(
+        string SourceFilePathVariable,
+        string DestinationFilePathVariable,
+        int Width,
+        int Height);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/TranscodeAudioArguments.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record TranscodeAudioArguments(
+        Guid TrackId,
+        string SourceFilePathVariable,
+        string WorkingDirectoryVariable);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/TransferFilesToBucketArguments.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record TransferFilesToBucketArguments(
+        string DestinationBucket,
+        string SourceDirectoryVariable,
+        string DestinationKey);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/UpdatePlayListPictureArguments.cs

@@ -0,0 +1,9 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record UpdatePlayListPictureArguments(
+        Guid PlayListId,
+        string OriginalPictureKey,
+        string SmallPictureVariable,
+        string MediumPictureVariable,
+        string LargePictureVariable);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/UpdateTrackAudioArguments.cs

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record UpdateTrackAudioArguments(
+        Guid TrackId,
+        string AudioFolderKey);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/UpdateTrackPictureArguments.cs

@@ -0,0 +1,9 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record UpdateTrackPictureArguments(
+        Guid TrackId,
+        string OriginalPictureKey,
+        string SmallPictureVariable,
+        string MediumPictureVariable,
+        string LargePictureVariable);
+}

backend/Musify.Infrastructure/MassTransit/Arguments/UploadFileToBucketArguments.cs

@@ -0,0 +1,7 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record UploadFileToBucketArguments(
+        string FilePathVariable,
+        string DestinationBucket,
+        string DestinationRoute);
+}