refactor(application): co-locate each command/query with its handler

One file per use case under the feature folder (Tracks/PlayLists/Users),
dropping the Commands/Queries/Handlers sub-folders and their namespaces.
Each file now holds the record and its handler in namespace
Musify.Application.<Feature>; external usings updated accordingly.

Build: 0 warnings, 0 errors.

backend/Musify.Api/Endpoints/UserEndpoints.cs

@@ -1,6 +1,5 @@
 using Mediator;
-using Musify.Application.Users.Commands;
-using Musify.Application.Users.Queries;
+using Musify.Application.Users;
 using Musify.Api.DataTransferObjects.Users;
 using Musify.Api.Extensions;
 using Musify.Api.Filters;