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/Logs/ConsumeUploadIntentsLog.cs

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

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

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

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

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

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

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

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

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

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

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

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

@@ -0,0 +1,6 @@
+namespace Musify.Infrastructure.MassTransit.Logs
+{
+    public record UpdateTrackAudioLog(
+        Guid TrackId,
+        string? PreviousAudioFolderName);
+}

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

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