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)

prisma/migrations/20260304152900_add_invoice_model/migration.sql

@@ -0,0 +1,69 @@
+-- CreateEnum
+CREATE TYPE "InvoiceStatus" AS ENUM ('DRAFT', 'SENT', 'PARTIAL', 'PAID', 'OVERDUE', 'VOID');
+
+-- CreateTable
+CREATE TABLE "Invoice" (
+    "id" TEXT NOT NULL,
+    "tenantId" TEXT NOT NULL,
+    "invoiceNumber" TEXT NOT NULL,
+    "subscriberId" TEXT NOT NULL,
+    "periodStart" TIMESTAMP(3) NOT NULL,
+    "periodEnd" TIMESTAMP(3) NOT NULL,
+    "dueDate" TIMESTAMP(3) NOT NULL,
+    "subtotal" DECIMAL(10,2) NOT NULL,
+    "totalAmount" DECIMAL(10,2) NOT NULL,
+    "amountPaid" DECIMAL(10,2) NOT NULL DEFAULT 0,
+    "status" "InvoiceStatus" NOT NULL DEFAULT 'DRAFT',
+    "journalEntryId" TEXT,
+    "issuedAt" TIMESTAMP(3),
+    "paidAt" TIMESTAMP(3),
+    "voidedAt" TIMESTAMP(3),
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "Invoice_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "InvoiceLine" (
+    "id" TEXT NOT NULL,
+    "tenantId" TEXT NOT NULL,
+    "invoiceId" TEXT NOT NULL,
+    "description" TEXT NOT NULL,
+    "quantity" INTEGER NOT NULL DEFAULT 1,
+    "unitPrice" DECIMAL(10,2) NOT NULL,
+    "lineTotal" DECIMAL(10,2) NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "InvoiceLine_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "Invoice_tenantId_idx" ON "Invoice"("tenantId");
+
+-- CreateIndex
+CREATE INDEX "Invoice_tenantId_status_idx" ON "Invoice"("tenantId", "status");
+
+-- CreateIndex
+CREATE INDEX "Invoice_tenantId_subscriberId_idx" ON "Invoice"("tenantId", "subscriberId");
+
+-- CreateIndex
+CREATE INDEX "Invoice_tenantId_dueDate_idx" ON "Invoice"("tenantId", "dueDate");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Invoice_tenantId_invoiceNumber_key" ON "Invoice"("tenantId", "invoiceNumber");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Invoice_tenantId_subscriberId_periodStart_key" ON "Invoice"("tenantId", "subscriberId", "periodStart");
+
+-- CreateIndex
+CREATE INDEX "InvoiceLine_invoiceId_idx" ON "InvoiceLine"("invoiceId");
+
+-- CreateIndex
+CREATE INDEX "InvoiceLine_tenantId_idx" ON "InvoiceLine"("tenantId");
+
+-- AddForeignKey
+ALTER TABLE "Invoice" ADD CONSTRAINT "Invoice_subscriberId_fkey" FOREIGN KEY ("subscriberId") REFERENCES "Subscriber"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "InvoiceLine" ADD CONSTRAINT "InvoiceLine_invoiceId_fkey" FOREIGN KEY ("invoiceId") REFERENCES "Invoice"("id") ON DELETE RESTRICT ON UPDATE CASCADE;