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.Api/Extensions/ErrorOrHttpExtensions.cs

@@ -0,0 +1,38 @@
+using ErrorOr;
+using IResult = Microsoft.AspNetCore.Http.IResult;
+
+namespace Musify.Api.Extensions;
+
+public static class ErrorOrHttpExtensions
+{
+    public static IResult ToHttpResult<T>(this ErrorOr<T> result)
+    {
+        if (result.IsError)
+            return Problem(result.Errors);
+
+        return result.Value is Success
+            ? Results.NoContent()
+            : Results.Ok(result.Value);
+    }
+
+    private static IResult Problem(List<Error> errors)
+    {
+        if (errors.All(e => e.Type == ErrorType.Validation))
+            return Results.ValidationProblem(ToValidationDictionary(errors));
+
+        var error = errors[0];
+        return error.Type switch
+        {
+            ErrorType.NotFound => Results.NotFound(error.Description),
+            ErrorType.Unauthorized => Results.Unauthorized(),
+            ErrorType.Forbidden => Results.StatusCode(StatusCodes.Status403Forbidden),
+            ErrorType.Conflict => Results.Conflict(error.Description),
+            _ => Results.Problem(error.Description)
+        };
+    }
+
+    private static Dictionary<string, string[]> ToValidationDictionary(IEnumerable<Error> errors) =>
+        errors
+            .GroupBy(e => string.IsNullOrEmpty(e.Code) ? "general" : e.Code)
+            .ToDictionary(g => g.Key, g => g.Select(e => e.Description).ToArray());
+}