The MVP data boundary
The current app processes return data in the browser and downloads the generated packet to the user's device. That keeps the first release away from server-side tax-return storage.
What a paid backend should know
A minimal paid backend should know who paid, what product/year they bought, when payment succeeded, and whether an entitlement is active. It should not store Form 709 payloads, generated PDFs, SSNs, addresses, names, or gift facts.
Why address autocomplete stays local
Third-party address autocomplete can transmit donor or donee addresses to another provider while a user types. Browser-native autofill and local state dropdowns keep the first workflow simpler and cleaner.
What changes later
Accounts, payments, e-file, professional review, and stored projects all raise the compliance bar. The product can add them, but each one should have a reviewed data map, logging policy, retention policy, and vendor boundary before launch.