Secure Web Rewrite With Paywall

I have an existing e-commerce application that now needs a complete overhaul. The goal is to turn it into a modern, browser-based solution that is significantly more secure while gating its core features behind a one-time-payment paywall.

Here’s what I’m after:
• Re-engineer the current codebase into a responsive web app (framework and language are up to you, as long as it is future-proof and well-documented).
• Build a solid authentication flow and weave the paywall directly into the sign-up / sign-in process so users pay once to unlock full access.
• Migrate all existing data, preserving current user accounts and purchase history.
• Implement standard best-practice hardening—encryption in transit and at rest, secure session handling, input sanitisation, and logging—while remaining open to additional suggestions like two-factor authentication or periodic security audits if they make sense.

When we’re done, I expect:
1. Source code with clear documentation.
2. A working deployment ready for production.
3. A brief walkthrough explaining how to manage the paywall (pricing changes, refunds, manual unlocks, etc.).

If you have a proven track record with e-commerce rebuilds and secure payment integration, I’d love to see how you would tackle this.