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/Authentication/JwtBearerEventsHandler.cs

@@ -1,6 +1,6 @@
 using Mediator;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
-using Musify.Application.Users.Commands;
+using Musify.Application.Users;
 using System.Security.Claims;
 
 namespace Musify.Api.Authentication;