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.

backend/Musify.Api/Endpoints/UserEndpoints.cs

@@ -59,7 +59,7 @@ public static class UserEndpoints
             new CreateUserCommand(request.Id, request.Name, request.FirstName, request.SecondName),
             cancellationToken);
 
-        if (!result.IsSuccess)
+        if (result.IsError)
             return result.ToHttpResult();
 
         return Results.Created($"/users/{result.Value.Id}", result.Value);