[2026-04-20][feat][general] refactor add-on domain, harden order/payment idempotency, and implement billing services

👤 Author : Matoraaa
🌿 Branch : master
🔗 Commit : 2d7a360
📅 Date : 2026-04-20
📝 Note :
feat: refactor add-on domain, harden order/payment idempotency, and implement billing services