Setup Amazon Pay (Amazon PS)

Routes

Route	Role
POST /api/amazon-ps/create-checkout	Create
POST /api/amazon-ps/launch	Launch
POST /api/amazon-ps/return	Return handler
POST /api/amazon-ps/verify-payment	Verify
POST /api/amazon-ps/webhook	Settlement

Enter Amazon PS merchant credentials in Payment methods.
Configure return URL to the Join Markt return route and webhook to /api/amazon-ps/webhook.
Test the full launch → return → verify path.