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.

backend/Musify.Application/AssemblyInfo.cs

@@ -0,0 +1,4 @@
+using Mediator;
+using Microsoft.Extensions.DependencyInjection;
+
+[assembly: MediatorOptions(ServiceLifetime = ServiceLifetime.Scoped)]