Setting up test website/app with integrations

I have a finished front-end for a new app/webapge. The whole app (UI and features) already exists and runs. I now need an experienced developer to add the "back-end" pieces that make it safe and ready for real users to both test and use. This is a well-scoped job with a clear written spec - not a build-from-scratch project. A complete React app exists. I do need: - Working backend deployed to accounts I own (integrations need to be set up with Anthropic; Cartesia; Voxtral and Stripe); - Front-end updated to use allow actual use with all secret keys removed from the browser At the end of the task I want to be able to properly test the app and website and then once happy, update and deploy. I have a guide to assist (prepared with Claude code). First time doing this, so open to reasonable offers and direction around price. An NDA will be required to be signed by the relevant developer and then file can be provided.