Place the chicken, potatoes, celery, onion, tomato paste, cumin and paprika in a large pot. Add the broth, cover and bring to a boil. Reduce heat, add ¾ tsp salt and ½ tsp pepper and simmer, covered, until the chicken is cooked through and the vegetables are tender, 15 to 20 minutes. (Skim and discard any foam that rises to the top.) Transfer the chicken to a bowl. When cool enough to handle, shred the meat, discarding the skin and bones. Stir the chicken back into the soup along with the chickpeas and couscous. Simmer for 3 minutes.