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

Musify.Infrastructure/MassTransit/Arguments/GenerateAudioWorkflowPathsArguments.cs

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