Add comprehensive guides for Stripe integration and purchase flow testing

Commit notes

- Introduced `PURCHASE_FLOW_TESTING.md` to document the complete marketplace purchase flow from creator onboarding to buyer checkout, including detailed steps and troubleshooting tips. - Added `STRIPE_SETUP.md` to outline the Stripe integration setup process, covering prerequisites, environment variables, webhook configuration, and testing procedures. - Enhanced backend API with new routes for analytics and improved handling of creator dashboard data. - Updated frontend routes and components to support new billing and checkout success/cancel pages, ensuring a seamless user experience during the purchase process. - Refactored analytics handling to support parallel data fetching for improved performance in the creator dashboard. - Cleaned up and organized code for better maintainability and clarity across the codebase.