edtech-billing-core

About

A framework-free TypeScript library on plain PostgreSQL covering lesson billing, payment webhooks and enumeration-safe login. Billing uses an append-only ledger with a fixed lock order so concurrent staff actions charge a student exactly once, and webhooks are verified by re-fetching the payment from the provider and inserting idempotently. It is extracted from the author's production school platform and tested against a real database with randomized invariant tests.