Full-stack developer to build the back-end for an existing website (auth, Stripe payments, database)

I'm the founder of EtheReal, an astrology compatibility platform. The front-end is fully built — a multi-page static site with the design, copy, and several forms already in place. I'm looking for a developer to build the back-end and bring it to launch. What's already done: the complete front-end (all pages, styling, responsive/mobile design), inert forms ready to be wired, and email is set up via Resend. What I need built: • User accounts / login & authentication (secure, with proper password handling). • A referral/rewards system (users refer others and progress through tiers). • Payment integration via Stripe for four flows: ebook purchase, paid astrologer readings, compatibility readings, and a "pay-what-you-want" supporter payment. The compatibility reading should default to a suggested €6, adjustable by the user, with a small minimum. I'll operate through an Estonian company, so Stripe + Stripe Tax for VAT. • A database for users, orders, referrals, and form submissions. • Form handling: Pure general/contact messages can simply be emailed to me (via Resend), no storage needed. The compatibility and booking forms, and the soulmate / "come forward" inquiry form, should be stored in the database with a simple admin view so I can browse, filter, and sort submissions (and delete them on request for GDPR). The compatibility/booking entries should tie into the payment and delivery flow. • Reading delivery: the compatibility reading is emailed to the buyer, then opens back on the site with a download option. • GDPR-compliant data handling (consent, secure storage, retention, deletion-on-request). • Hosting setup and secure deployment. • Basic admin tools so I can manage orders, users, and submissions without a developer. Critical requirement — the algorithm: the platform runs on a proprietary algorithm central to the business. It must stay server-side and protected, accessed only as a black-box API (the system calls it for results; nothing exposes its internal logic). The system must be architected so I can deploy the real algorithm file myself, via a simple documented process — it should never need to pass through your hands. During development you'd build and test against a dummy placeholder with the same inputs/outputs. To start, I'd like a small paid trial task before the full engagement: build the user login/signup flow (secure auth, not yet connected to the rest of the system). This lets us see how we work together before committing to the whole project. Please reply with: 1. Your approach to keeping the algorithm secure and how you'd structure the black-box API. 2. How you'd handle secure authentication and password storage. 3. Relevant past work (live sites, repos, or references I can contact). 4. Your estimate and rate for the full build, ideally broken into milestones. 5. Your availability, and what post-launch support looks like (bug-fix period, ongoing maintenance, and rates). All work and IP will belong to me, and I'll ask for an NDA before any access to sensitive parts. The current front-end is here: https://ethe-real.netlify.app/ — note this is front-end only; the back-end is what I'm hiring for. Thanks — looking forward to hearing how you'd approach it.