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.Api/DataTransferObjects/Tracks/RequestTrackUploadUrlsRequest.cs

@@ -0,0 +1,9 @@
+namespace Musify.Api.DataTransferObjects.Tracks;
+
+public record RequestTrackUploadUrlsRequest(
+    string PictureFileType,
+    string PictureContentType,
+    string AudioFileType,
+    string AudioContentType,
+    long? ExpectedPictureSizeBytes = null,
+    long? ExpectedAudioSizeBytes = null);