refactor(application): fold Pagination into Shared, rename assembly marker

- Move PaginatedResponse from the single-file Pagination/ folder into Shared/
  (namespace Musify.Application.Pagination -> Musify.Application.Shared)
- Rename MusifyApplication.cs -> AssemblyInfo.cs (it only holds the
  [assembly: MediatorOptions] attribute)

Build: 0 warnings, 0 errors.

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

backend/Musify.Application/Tracks/GetTracks.cs

@@ -1,7 +1,7 @@
 using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
-using Musify.Application.Pagination;
+using Musify.Application.Shared;
 using Musify.Application.Tracks.Responses;
 using X.PagedList.EF;
 using Musify.Application.Contracts;