SaaS Billing & Subscriptions Platform

Designed and shipped the billing core of a multi-tenant SaaS end to end. Modeled subscriptions, usage records, and invoices in PostgreSQL; built an idempotent worker that aggregates usage events and reconciles them against Stripe webhooks; and wrote the React billing portal where customers upgrade, downgrade, and view line-item invoices. The hardest part was making it exactly-once: every webhook is deduplicated, every state transition is append-only, and the reconciliation job proves the ledger balances every night.