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/Pagination/PaginatedResponse.cs

@@ -0,0 +1,26 @@
+using X.PagedList;
+
+namespace Musify.Application.Pagination
+{
+    public record PaginatedResponse<T>(
+        IReadOnlyCollection<T> Items,
+        int PageNumber,
+        int PageSize,
+        int PageCount,
+        int TotalItemCount,
+        bool HasNextPage,
+        bool HasPreviousPage)
+    {
+        public static PaginatedResponse<T> FromPagedList(IPagedList<T> pagedList)
+        {
+            return new PaginatedResponse<T>(
+                pagedList.ToArray(),
+                pagedList.PageNumber,
+                pagedList.PageSize,
+                pagedList.PageCount,
+                pagedList.TotalItemCount,
+                pagedList.HasNextPage,
+                pagedList.HasPreviousPage);
+        }
+    }
+}