Integrate a payment gateway into our existing web application. The implementation must support secure payment initiation, success and failure handling, verification callbacks, transaction records, and clear user messaging. We need a production-minded integration with proper testing and documentation rather than a basic demo. The successful freelancer will be expected to work independently, share progress at agreed checkpoints, and raise questions early when a requirement needs clarification. All submitted work should be original, well organized, and ready for practical use after reasonable review.