feat: bring over the albums, explore and player work from the worktree

Moves the remaining uncommitted work: the home gets quick mixes and a
"Tus álbumes" shelf, saving a YouTube album now goes through
addYoutubeAlbumToPlaylistAction instead of the SaveYouTubeAlbum command,
and UserAlbum drops its unused ExternalId.

The mixes section keeps its current shape (no refresh action, no badge,
hidden when empty) rather than the older version the worktree carried.

backend/Musify.Infrastructure/Persistence/Database.cs

@@ -111,10 +111,6 @@ namespace Musify.Infrastructure.Persistence
             modelBuilder.Entity<UserAlbum>()
                 .ToTable("UserAlbums");
 
-            modelBuilder.Entity<UserAlbum>()
-                .HasIndex(album => new { album.OwnerUserId, album.ExternalId })
-                .IsUnique();
-
             modelBuilder.Entity<UserAlbum>()
                 .HasOne(album => album.Owner)
                 .WithMany()