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/Users/Handlers/GetUserByIdQueryHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
@@ -8,9 +8,9 @@ using Musify.Application.Users.Responses;
 namespace Musify.Application.Users.Handlers
 {
     public class GetUserByIdQueryHandler(IDatabase database)
-        : IQueryHandler<GetUserByIdQuery, Result<UserApplicationResponse> >
+        : IQueryHandler<GetUserByIdQuery, ErrorOr<UserApplicationResponse> >
     {
-        public async ValueTask<Result<UserApplicationResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<UserApplicationResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
         {
             var user = await database.Users
                 .AsNoTracking()
@@ -18,9 +18,10 @@ namespace Musify.Application.Users.Handlers
                 .Select(u => UserApplicationResponse.FromEntity(u))
                 .SingleOrDefaultAsync(cancellationToken);
 
-            return user is null
-                ? Result<UserApplicationResponse>.NotFound()
-                : Result.Success(user);
+            if (user is null)
+                return Error.NotFound();
+
+            return user;
         }
     }
 }