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/TrackApplicationResponse.cs

@@ -0,0 +1,20 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackApplicationResponse(
+        Guid Id,
+        string Title,
+        DateTime CreatedAt,
+        DateTime UpdatedAt)
+    {
+        public static TrackApplicationResponse FromEntity(Track track)
+        {
+            return new TrackApplicationResponse(
+                track.Id,
+                track.Title,
+                track.CreatedAt,
+                track.UpdatedAt);
+        }
+    }
+}