feat(02-04): Invoice model, BillingService, InvoiceService, CreditService
- Add InvoiceStatus enum + Invoice + InvoiceLine models to schema
- Add Invoice relation to Subscriber model
- Add invoice/invoiceLine to TENANT_SCOPED_MODELS in prisma-tenant.ts
- Add invoice/invoiceLine tenant-scoped query extensions to withTenantContext()
- Run migration: 20260304152900_add_invoice_model
- Create invoice-service.ts: generateInvoiceNumber, getInvoice, listInvoices,
updateInvoiceStatus, markOverdueInvoices, voidInvoice (with JE reversal)
- Create credit-service.ts: applyCredit() — DR Subscriber Credits (1150), CR AR (1100)
- Create billing-service.ts: generateInvoiceForSubscriber (idempotent, JE + auto-credit),
generateMonthlyInvoices (postpaid + prepaid timing with lead days)
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