Initialize repo and add session changes (2025-09-08)

docs/prp/08-billing.md

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+# 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.