Phase 1: Created 6 billing tables (billing_plans, billing_overage_rates, billing_subscriptions, billing_invoices, billing_invoice_items, billing_usage_snapshots). Seeded 3 plans   5 overage rates. Migrated 167 sites to billing_subscriptions. Added 30  CRUD methods to billing.class.php. Added invoice generation engine to BillingService.php (generateInvoice, calculateOverages, applyProration, processAutoPayment, getTenantBillingSummary, getUsageMeters). Added 20  controller actions (tenant: getBillingOverview, getInvoices, getInvoiceDetail, getSubscriptionDetails, getUsageMeters, payInvoice, updateBillingCycle, updateAutoPay; admin: adminGetBillingDashboard, adminGenerateInvoice, adminVoidInvoice, adminManagePlans, adminManageOverageRates). Phase 3: Created /platform/services/billing/ module with 8 pages (overview, invoices, invoice_detail, subscription, usage, payment_methods, payment_history, reports), pagedata.php, billing_js.php, billing.css. Added Billing to sidebar between Web Pages and Web Services in menu_administration.php.