08 — Billing (MVP: Event Credits)

Model: one-off purchases that grant event credits; no subscriptions in MVP.
Tables: event_purchases, event_credits_ledger (see 04-data-model-migrations.md).
Providers: Stripe (server-side checkout + webhooks); store receipts deferred.
Idempotency: purchase intents keyed; ledger writes idempotent; retries safe.
Limits: enforce event_credits_balance >= 1 to create an event; ledger decrements on event creation.