feat(backend): add playlist cover endpoint and relax description validation

Adds GET /playlists/{id}/cover (mirrors the track cover endpoint) so
uploaded playlist pictures can actually be served. Also drops the
NotEmpty rule on playlist description, which rejected the blank
description the frontend treats as optional; CreatePlayList now falls
back to the domain's default text instead.

backend/Musify.Api/Validators/PlayLists/UpdatePlayListRequestValidator.cs

@@ -13,7 +13,6 @@ public sealed class UpdatePlayListRequestValidator : AbstractValidator<UpdatePla
             .When(x => x.NewName is not null);
 
         RuleFor(x => x.NewDescription)
-            .NotEmpty()
             .MaximumLength(256)
             .When(x => x.NewDescription is not null);
     }