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/Tracks/Handlers/GetTrackByIdQueryHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
@@ -8,18 +8,19 @@ using Musify.Application.Tracks.Responses;
 namespace Musify.Application.Tracks.Handlers
 {
     public class GetTrackByIdQueryHandler(IDatabase database)
-        : IQueryHandler<GetTrackByIdQuery, Result<TrackApplicationResponse> >
+        : IQueryHandler<GetTrackByIdQuery, ErrorOr<TrackApplicationResponse> >
     {
-        public async ValueTask<Result<TrackApplicationResponse>> Handle(GetTrackByIdQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<TrackApplicationResponse>> Handle(GetTrackByIdQuery request, CancellationToken cancellationToken)
         {
             var track = await database.Tracks.AsNoTracking()
                 .Where(t => t.Id == request.TrackId)
                 .Select(t => TrackApplicationResponse.FromEntity(t))
                 .SingleOrDefaultAsync(cancellationToken);
 
-            return track is null
-                ? Result<TrackApplicationResponse>.NotFound()
-                : Result.Success(track);
+            if (track is null)
+                return Error.NotFound();
+
+            return track;
         }
     }
 }