Every Friday and Saturday, black velvet curtains are drawn to cover the soda fountain at Della’s Delectables to create a wall with the elegant simplicity of a Chanel couture frock. A swirl of fairy-godparents places crisp black linens on all the tables, plus gleaming crystal and shining cutlery, to transform the modest storefront. Wasn’t this just a deli at 3 p.m.? Now it’s a jazz club with “Dolce Debbie” Frangipane’s four-star cuisine.