For the stew: Heat the oil in a Dutch oven over medium-high heat. Add the celery, thyme, paprika, bay leaves, carrots, onions, bell peppers, chili peppers and saffron and season with salt and pepper. Cook until the vegetables are soft, about 8 minutes. Add the tomatoes and mash. Add the stock and simmer for 30 minutes. Stir in the chicken. Stir in the picada and olives, then remove from the heat and let stand for 5 minutes.