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.Application/PlayLists/Handlers/GetPlayListsQueryHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Pagination;
 using Musify.Application.PlayLists.Responses;
@@ -10,9 +10,9 @@ using Musify.Application.Contracts;
 namespace Musify.Application.PlayLists.Handlers
 {
     public class GetPlayListsQueryHandler(IDatabase database)
-        : IQueryHandler<GetPlayListsQuery, Result<PaginatedResponse<PlayListApplicationResponse> >>
+        : IQueryHandler<GetPlayListsQuery, ErrorOr<PaginatedResponse<PlayListApplicationResponse> >>
     {
-        public async ValueTask<Result<PaginatedResponse<PlayListApplicationResponse> >> Handle(GetPlayListsQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<PaginatedResponse<PlayListApplicationResponse> >> Handle(GetPlayListsQuery request, CancellationToken cancellationToken)
         {
             var totalCount = await database.PlayLists.CountAsync(cancellationToken);
 
@@ -22,7 +22,7 @@ namespace Musify.Application.PlayLists.Handlers
                 .Select(p => PlayListApplicationResponse.FromEntity(p))
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
-            return Result.Success(PaginatedResponse<PlayListApplicationResponse>.FromPagedList(pagedPlayLists));
+            return PaginatedResponse<PlayListApplicationResponse>.FromPagedList(pagedPlayLists);
         }
     }
 }