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/PlayLists/Commands/AddTrackToPlayListCommand.cs

@@ -0,0 +1,8 @@
+using Ardalis.Result;
+using Mediator;
+
+namespace Musify.Application.PlayLists.Commands
+{
+    public record AddTrackToPlayListCommand(long UserId, Guid PlayListId, Guid TrackId)
+        : ICommand<Result>;
+}