refactor: replace Ardalis.Result with ErrorOr across the app

Ardalis Result<T> carried incoherent success sub-statuses and a clunky
generic converter; ErrorOr models success-or-errors cleanly.

- Application: all commands/queries/handlers now return ErrorOr<T>
  (non-generic Result -> ErrorOr<Success>); errors via Error.NotFound/
  Conflict/Validation/Unauthorized/Failure; success via value or new Success()
- Drop ResultExtensions.As<>(): error propagation is now just return .Errors
- Rewrite HTTP mapping (ErrorOrHttpExtensions) off Error.Type; success maps
  to 200 (value) or 204 (Success); Created still built in the endpoints
- Endpoints/gateway middleware: !IsSuccess -> IsError
- Swap Ardalis.Result package for ErrorOr 2.1.1 (remove unused ref from Worker)

Build: 0 warnings, 0 errors.

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

backend/Musify.Application/PlayLists/Commands/RemoveTrackFromPlayListCommand.cs

@@ -1,8 +1,8 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 
 namespace Musify.Application.PlayLists.Commands
 {
     public record RemoveTrackFromPlayListCommand(long UserId, Guid PlayListId, Guid TrackId)
-        : ICommand<Result>;
+        : ICommand<ErrorOr<Success>>;
 }