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/Responses/TrackUploadUrlsResponse.cs

@@ -0,0 +1,16 @@
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackUploadUrlsResponse(
+        Guid PictureIntentId,
+        Guid AudioIntentId,
+        string Bucket,
+        string PictureKey,
+        string PictureName,
+        string PictureContentType,
+        string PictureUploadUrl,
+        string AudioKey,
+        string AudioName,
+        string AudioContentType,
+        string AudioUploadUrl,
+        int ExpiresInSeconds);
+}