The work to process payments with Swish should be initially implemented in the PoC tracked by #3942. We need to refine and integrate this functionality properly by cleaning up the existing code, placing it behind the feature flag #3973, and adding test coverage.

• Add Swish behind the feature flag to allow controlled rollouts.
• Clean up the code from the PoC branch to align with best practices and maintainability.
• Implement unit tests to cover the payment processing logic.
• Ensure a simple product can be successfully purchased using Swish without saving it.