Created Billing_System_Final_Plan.md — single source of truth merging v1, v2, and architecture analysis. Builds on existing stripe_*, BillingService, StripeService, CouponService. 6 new tables (billing_plans, billing_overage_rates, billing_subscriptions, billing_invoices, billing_invoice_items, billing_usage_snapshots). 3-layer model: setup fees   recurring   overages. 6 implementation phases, 12-18 days.