Thaw the lobster in the refrigerator the day before. Drain and reserve the liquid. Remove all cartilage fragments and chop the meat. Melt the butter in a large pot. Add the celery, onion, and carrot, and cook until tender. Sprinkle with flour. Stir and cook for 1 to 2 minutes. Gradually add the milk, stirring constantly. Continue to stir until the mixture thickens. Season with salt and pepper. Pour in the broth. Purée the vegetables with a mixer. Return the broth and vegetables to the pot. Add the reserved liquid and the lobster meat. Let it simmer for a few minutes to heat everything through. Add the lemon juice and stir. Sprinkle with a few grains of Cayenne pepper and paprika. Cut the bread into cubes. Toast under the broiler. Add the croutons to the bisque when serving.