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/GetPlayListTracksQueryHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
@@ -10,15 +10,15 @@ using X.PagedList.EF;
 namespace Musify.Application.PlayLists.Handlers
 {
     public class GetPlayListTracksQueryHandler(IDatabase database)
-        : IQueryHandler<GetPlayListTracksQuery, Result<PaginatedResponse<TrackApplicationResponse>> >
+        : IQueryHandler<GetPlayListTracksQuery, ErrorOr<PaginatedResponse<TrackApplicationResponse>> >
     {
-        public async ValueTask<Result<PaginatedResponse<TrackApplicationResponse> >> Handle(GetPlayListTracksQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<PaginatedResponse<TrackApplicationResponse> >> Handle(GetPlayListTracksQuery request, CancellationToken cancellationToken)
         {
             var playListExists = await database.PlayLists
                 .AsNoTracking()
                 .AnyAsync(p => p.Id == request.PlayListId, cancellationToken);
             if (!playListExists)
-                return Result<PaginatedResponse<TrackApplicationResponse>>.NotFound();
+                return Error.NotFound();
 
             var tracksQuery = database.PlayListHasTracks
                 .AsNoTracking()
@@ -32,7 +32,7 @@ namespace Musify.Application.PlayLists.Handlers
                 .Select(plt => TrackApplicationResponse.FromEntity(plt.Track))
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
-            return Result.Success(PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks));
+            return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
         }
     }
 }