In a large Dutch oven, warm the olive oil over medium heat. Add the celery, bell pepper, onion, and carrot and cook, stirring, until soft, about 12 minutes. Add the tomato paste and cook, stirring, for 2 minutes. Increase the heat to high, add the tomatoes, garlic, and zucchini, and cook, stirring, until they start to brown lightly, about 2 minutes. Pour in the fish stock and saffron and bring to a boil.