Muouzik is a digital music and video marketplace built for artists and listeners in Guinea and West Africa. I designed and built the entire backend (a versioned NestJS REST API with 13 feature modules and two Socket.IO gateways) and the Next.js admin portal used to run the platform. Listeners buy songs and albums and stream them through presigned S3 URLs gated by digital ownership; artists upload albums atomically in a single API call, track royalties and request payouts; admins moderate content, verify creators, manage coupons and referrals, and approve payouts. Payments run through ChapChapPay (Orange Money, MTN MoMo, PayCard, cards) with webhook-based fulfillment and mobile deep-link callbacks. The Flutter mobile app (Android & iOS) was built by my teammate on top of this API.