NumDojo — Speed Math Learning Platform

NumDojo is where I put my name on the line as a builder, not just a reviewer. One person, the full stack: Next.js and TypeScript on the frontend, Node.js and Express on the backend, PostgreSQL with a 50+ table schema, Redis for caching, Socket.io for real-time play, Stripe for subscriptions, all running in Docker. The hard parts were the systems most products never need. An SM-2 spaced-repetition queue that reschedules every answered problem until it is mastered. Live 1v1 duels with ELO-based matchmaking. Auto-scheduled tournaments with brackets and leagues. Usage-based subscription limits that have to stay correct across Stripe billing events. I built it the way I audit: JWT auth, bcrypt hashing, parameterized queries everywhere, rate limiting on sensitive endpoints, and COPPA compliance because the audience includes kids. An auditor's own product is a standing audit target, and I treated it like one.