refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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

Musify.Application/Tracks/Commands/RequestTrackUploadUrlsCommand.cs

@@ -0,0 +1,16 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks.Commands
+{
+    public record RequestTrackUploadUrlsCommand(
+        long UserId,
+        string PictureFileType,
+        string PictureContentType,
+        string AudioFileType,
+        string AudioContentType,
+        long? ExpectedPictureSizeBytes = null,
+        long? ExpectedAudioSizeBytes = null)
+        : ICommand<Result<TrackUploadUrlsResponse>>;
+}