Add login and tiered access to an existing web app (Supabase + Vercel)

I have a working web app, a single interactive HTML page hosted on Vercel, with its content in a separate JavaScript file. I need a developer to add user login and two access tiers. The app and its content already exist and will not change. This is purely about adding authentication and gating. How access should work: – Anyone can use the app without logging in. They see the free "Lite" tier. – A user can sign in with their email using passwordless login (a one-time email link or code). Please use Supabase Auth, which the app is already set up to use. – If the signed-in email is on an NHS domain (nhs.uk, nhs.net, and a short list I will provide), the user gets the full "Full" tier for free. – If the signed-in email is not an NHS domain, they stay on the Lite tier unless I have manually marked them as paid in the database. There is no payment system to build. I grant paid access by hand for now. – The app currently has a temporary on-screen toggle that switches between Lite and Full. This must be removed and replaced by the login logic, so the tier is decided by who is signed in, not by a button. There is also an existing "non-NHS access options" page in the app. Non-NHS users who are not signed in, or not marked as paid, should be able to reach it. NHS users do not need it. What I will provide: – The current app files (the HTML page and the content file). – Access to the Vercel project and the Supabase project, or I will set up access as you advise. – The list of NHS email domains to treat as free. What I need back: – The updated app with login working and the two tiers gated correctly. – A simple way for me to mark a non-NHS user as paid, and to remove that, without needing a developer each time. A basic admin view or clear instructions for doing it in Supabase is fine. – Short written notes on how it works and how to maintain it. About the stack, so you can judge fit: the app is a static site on Vercel, deployed from a GitHub repo. It already uses Supabase. I am comfortable committing files to GitHub and using the Vercel and Supabase dashboards, but I am not a professional developer, so I need the work done cleanly and documented simply. Please quote a fixed price for the whole job, and tell me roughly how long it will take. In your reply, tell me briefly about a similar auth or access-gating job you have done with Supabase.