feat(annotation): Add Origin/Width/Height parameters to AddAnnotationCommand

- Add Origin parameter (default: BottomLeft)
- Add Width/Height as alternatives to X2/Y2
- Validation: Either (X2+Y2) OR (Width+Height) required, not both
- FluentValidation rules enforce mutual exclusivity

DocumentOperator.Application/AddAnnotation/AddAnnotationCommand.cs

@@ -18,6 +18,7 @@ public record AddAnnotationCommand : IRequest<byte[]>
     public string? Author { get; init; }
     public string? Color { get; init; }
     public TextMarkupStyle? TextMarkupStyle { get; init; }
+    public AnnotationOrigin Origin { get; init; } = AnnotationOrigin.BottomLeft;
 }
 
 /// <summary>
@@ -35,7 +36,8 @@ public class AddAnnotationHandler(IPdfProcessor pdfProcessor) : IRequestHandler<
             request.Content,
             request.Author,
             request.Color,
-            request.TextMarkupStyle);
+            request.TextMarkupStyle,
+            request.Origin);
     }
 }