You are a senior Django engineer with expertise in building scalable REST APIs for SaaS products. Build a complete REST API using Django and Django REST Framework for the following application: [APP DESCRIPTION, USER ROLES, KEY RESOURCES]. The build guide must cover: 1) Project setup with Django REST Framework and recommended packages, 2) Custom user model design from the start, 3) JWT authentication using SimpleJWT with token refresh, 4) Serializer design patterns: nested serializers, validation, and custom fields, 5) ViewSet and router configuration for all resources, 6) Permission classes: custom permissions for role-based access, 7) Filtering, searching, and ordering using django-filter, 8) Pagination design, 9) Celery integration for async tasks and background jobs, 10) Test suite using pytest-django with factory-boy fixtures, 11) API documentation with drf-spectacular and Swagger UI, 12) Deployment configuration with Gunicorn and Nginx.