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/PlayListApplicationResponse.cs

@@ -0,0 +1,28 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.PlayLists.Responses
+{
+    public record PlayListApplicationResponse(
+        Guid Id,
+        string Name,
+        string Description,
+        string SmallImageKeyName,
+        string MediumImageKeyName,
+        string LargeImageKeyName,
+        DateTime CreatedAt,
+        DateTime UpdatedAt)
+    {
+        public static PlayListApplicationResponse FromEntity(PlayList playList)
+        {
+            return new PlayListApplicationResponse(
+                playList.Id,
+                playList.Name,
+                playList.Description,
+                playList.SmallPictureName,
+                playList.MediumPictureName,
+                playList.LargePictureName,
+                playList.CreatedAt,
+                playList.UpdatedAt);
+        }
+    }
+}

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);
+}