Add billing documentation for v2.9.0 New pages: - Billing-Integration.md: Quick start guide - Purchase-Categories.md: CONSUMABLE, FEATURE_UNLOCK, LIFETIME_PREMIUM, REMOVE_ADS - Consumable-Products.md: Manual consumption flow - Subscriptions.md: Subscription lifecycle states - Subscription-Upgrades.md: Upgrade/downgrade with proration modes 🤖 Generated with [Claude Code](https://claude.com/claude-code)