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/Commands/CreateUserCommand.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Musify.Application.Users.Responses;
 using Musify.Domain.Entities;
@@ -9,7 +9,7 @@ namespace Musify.Application.Users.Commands
         string Name,
         string? FirstName,
         string? SecondName)
-        : ICommand<Result<UserApplicationResponse>>
+        : ICommand<ErrorOr<UserApplicationResponse>>
     {
         public static User ToEntity(CreateUserCommand command)
         {

backend/Musify.Application/Users/Commands/SyncUserCommand.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 
 namespace Musify.Application.Users.Commands
@@ -9,5 +9,5 @@ namespace Musify.Application.Users.Commands
         string? FirstName,
         string? SecondName,
         string? ProfilePictureUrl)
-        : ICommand<Result>;
+        : ICommand<ErrorOr<Success>>;
 }

backend/Musify.Application/Users/Handlers/CreateUserCommandHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
@@ -9,15 +9,15 @@ using Musify.Application.Users.Responses;
 namespace Musify.Application.Users.Handlers
 {
     public class CreateUserCommandHandler(IDatabase database, ILogger<CreateUserCommandHandler> logger)
-        : ICommandHandler<CreateUserCommand, Result<UserApplicationResponse> >
+        : ICommandHandler<CreateUserCommand, ErrorOr<UserApplicationResponse> >
     {
-        public async ValueTask<Result<UserApplicationResponse>> Handle(CreateUserCommand request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<UserApplicationResponse>> Handle(CreateUserCommand request, CancellationToken cancellationToken)
         {
             var userExists = await database.Users.AnyAsync(u => u.Id == request.Id, cancellationToken);
             if (userExists)
             {
                 logger.LogInformation("User {UserId} already exists", request.Id);
-                return Result<UserApplicationResponse>.Conflict($"User {request.Id} already exists");
+                return Error.Conflict(description: $"User {request.Id} already exists");
             }
 
             var newUser = CreateUserCommand.ToEntity(request);
@@ -30,11 +30,11 @@ namespace Musify.Application.Users.Handlers
             catch (Exception exception)
             {
                 logger.LogError(exception, "Failed to create user {UserId}", request.Id);
-                return Result.Error($"Failed to create user {request.Id}");
+                return Error.Failure(description: $"Failed to create user {request.Id}");
             }
 
             logger.LogInformation("Created user {UserId}", newUser.Id);
-            return Result.Created(UserApplicationResponse.FromEntity(newUser));
+            return UserApplicationResponse.FromEntity(newUser);
         }
     }
 }

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;
         }
     }
 }

backend/Musify.Application/Users/Handlers/GetUsersQueryHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Pagination;
@@ -10,9 +10,9 @@ using Musify.Application.Contracts;
 namespace Musify.Application.Users.Handlers
 {
     public class GetUsersQueryHandler(IDatabase database)
-        : IQueryHandler<GetUsersQuery, Result<PaginatedResponse<UserApplicationResponse> > >
+        : IQueryHandler<GetUsersQuery, ErrorOr<PaginatedResponse<UserApplicationResponse> > >
     {
-        public async ValueTask<Result<PaginatedResponse<UserApplicationResponse> >> Handle(GetUsersQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<PaginatedResponse<UserApplicationResponse> >> Handle(GetUsersQuery request, CancellationToken cancellationToken)
         {
             var user = database.Users
                 .AsNoTracking()
@@ -29,7 +29,7 @@ namespace Musify.Application.Users.Handlers
                 .Select(u => UserApplicationResponse.FromEntity(u))
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
-            return Result.Success(PaginatedResponse<UserApplicationResponse>.FromPagedList(pagedUser));
+            return PaginatedResponse<UserApplicationResponse>.FromPagedList(pagedUser);
         }
     }
 }

backend/Musify.Application/Users/Handlers/SyncUserCommandHandler.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
@@ -9,9 +9,9 @@ using Musify.Domain.Entities;
 namespace Musify.Application.Users.Handlers
 {
     public class SyncUserCommandHandler(IDatabase database, ILogger<SyncUserCommandHandler> logger)
-        : ICommandHandler<SyncUserCommand, Result>
+        : ICommandHandler<SyncUserCommand, ErrorOr<Success>>
     {
-        public async ValueTask<Result> Handle(SyncUserCommand request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<Success>> Handle(SyncUserCommand request, CancellationToken cancellationToken)
         {
             var user = await database.Users
                 .SingleOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
@@ -31,7 +31,7 @@ namespace Musify.Application.Users.Handlers
                 await database.SaveChangesAsync(cancellationToken);
                 logger.LogInformation("Provisioned user {UserId}", request.Id);
 
-                return Result.Success();
+                return new Success();
             }
 
             user.Name = request.Name;
@@ -44,7 +44,7 @@ namespace Musify.Application.Users.Handlers
             if (written > 0)
                 logger.LogInformation("Synced profile for user {UserId}", request.Id);
 
-            return Result.Success();
+            return new Success();
         }
     }
 }

backend/Musify.Application/Users/Queries/GetUserByIdQuery.cs

@@ -1,9 +1,9 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Musify.Application.Users.Responses;
 
 namespace Musify.Application.Users.Queries
 {
     public record GetUserByIdQuery(long Id)
-        : IQuery<Result<UserApplicationResponse> >;
+        : IQuery<ErrorOr<UserApplicationResponse> >;
 }

backend/Musify.Application/Users/Queries/GetUsersQuery.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Musify.Application.Pagination;
 using Musify.Application.Users.Responses;
@@ -6,5 +6,5 @@ using Musify.Application.Users.Responses;
 namespace Musify.Application.Users.Queries
 {
     public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch = null)
-        : IQuery<Result<PaginatedResponse<UserApplicationResponse> >>;
+        : IQuery<ErrorOr<PaginatedResponse<UserApplicationResponse> >>;
 }