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/Responses/PlayListPictureUploadResponse.cs

@@ -0,0 +1,11 @@
+namespace Musify.Application.PlayLists.Responses
+{
+    public record PlayListPictureUploadResponse(
+        Guid IntentId,
+        string Bucket,
+        string Key,
+        string PictureName,
+        string ContentType,
+        int ExpiresInSeconds,
+        string UploadUrl);
+}