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/Tracks/Commands/CreateTrackCommand.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Musify.Application.Tracks.Responses;
 
@@ -9,5 +9,5 @@ namespace Musify.Application.Tracks.Commands
         string Title,
         Guid PictureIntentId,
         Guid AudioIntentId)
-        : ICommand<Result<TrackApplicationResponse>>;
+        : ICommand<ErrorOr<TrackApplicationResponse>>;
 }

backend/Musify.Application/Tracks/Commands/DeleteTrackCommand.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 
 namespace Musify.Application.Tracks.Commands
@@ -6,5 +6,5 @@ namespace Musify.Application.Tracks.Commands
     public record DeleteTrackCommand(
         long UserId,
         Guid TrackId)
-        : ICommand<Result>;
+        : ICommand<ErrorOr<Success>>;
 }

backend/Musify.Application/Tracks/Commands/RequestTrackUploadUrlsCommand.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Mediator;
 using Musify.Application.Tracks.Responses;
 
@@ -12,5 +12,5 @@ namespace Musify.Application.Tracks.Commands
         string AudioContentType,
         long? ExpectedPictureSizeBytes = null,
         long? ExpectedAudioSizeBytes = null)
-        : ICommand<Result<TrackUploadUrlsResponse>>;
+        : ICommand<ErrorOr<TrackUploadUrlsResponse>>;
 }