Full-Stack Developer — Geolocated Scavenger Hunt Web App (Accounts, Interactive Map, Stripe Payment)

We're building a mobile-first web application for a location-based scavenger hunt / trivia game (think Pokémon Go meets a trivia quiz). Users create an account, follow an interactive map locked to their active trail, unlock quiz questions by GPS proximity to real-world points of interest, and can purchase additional themed trails. This is a fixed-price project delivered in 4 milestones (full spec attached). We need a reliable developer who delivers a testable, working demo at each milestone — not just code we can't evaluate ourselves. SCOPE OF WORK Milestone 1 — Foundations - User accounts (magic-link email login, no password) - Interactive map (Leaflet/OpenStreetMap), locked to the active trail's zone only - GPS proximity detection + reference photo confirmation to unlock each stop - Quiz engine (multiple choice, progress saved, resumable) - Content admin back-office (add towns/trails/questions without touching code) - Mobile-first UI design Milestone 2 — Deployment - Hosting, domain, testing — live and usable for a real-world pilot test Milestone 3 — Monetization - Stripe payment integration for purchasing additional trails - Digital passport with unlockable badges - Leaderboard (by town / global / monthly / annual, pseudonym only — no real names) - Automatic loyalty discounts after N completed trails - Group/birthday mode (shareable session code, no account needed for guests) Milestone 4 — Handover - Full source code transferred to our own GitHub repository - Minimal documentation (how to add a trail, how to redeploy) - Short walkthrough call of the admin back-office REQUIRED SKILLS JavaScript/TypeScript, React (or similar), Node.js, PostgreSQL or similar, Leaflet/OpenStreetMap or similar mapping library, Stripe integration, responsive mobile-first design. Prior experience with geolocation-based apps is a strong plus. BUDGET Fixed price, depending on experience and proposal, split across the 4 milestones above, paid via Upwork escrow per milestone. To apply: please start your proposal with the word "Trailhead" so I know you've read the full brief, and briefly describe a past project involving accounts + maps + online payment.