chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/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);
+        }
+    }
+}

backend/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);
+}