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/schema.prisma

@@ -81,6 +81,15 @@ enum JournalEntrySource {
   MANUAL
 }
 
+enum InvoiceStatus {
+  DRAFT
+  SENT
+  PARTIAL
+  PAID
+  OVERDUE
+  VOID
+}
+
 // =============================================================================
 // MODELS
 // =============================================================================
@@ -236,6 +245,8 @@ model Subscriber {
   createdAt     DateTime         @default(now())
   updatedAt     DateTime         @updatedAt
 
+  invoices      Invoice[]
+
   /// Account numbers must be unique within a tenant
   @@unique([tenantId, accountNumber])
   /// RLS-ready index — always present on tenant-scoped models
@@ -345,3 +356,68 @@ model JournalEntryLine {
   @@index([accountId])
   @@index([journalEntryId])
 }
+
+/// An Invoice is a billing document issued to a subscriber for a billing period.
+/// Invoices are generated by the BillingService (auto) or manually.
+/// Each invoice generation creates a balanced journal entry (DR AR, CR Revenue).
+/// amountPaid is a transactional convenience field — always updated atomically with JEs.
+model Invoice {
+  id            String        @id @default(uuid())
+  tenantId      String
+  /// Auto-generated sequential identifier per tenant (e.g., "INV-2026-0001")
+  invoiceNumber String
+  subscriberId  String
+  subscriber    Subscriber    @relation(fields: [subscriberId], references: [id])
+  /// Billing period start date (inclusive)
+  periodStart   DateTime
+  /// Billing period end date (inclusive)
+  periodEnd     DateTime
+  /// Payment due date
+  dueDate       DateTime
+  /// Subtotal before any adjustments (sum of line totals)
+  subtotal      Decimal       @db.Decimal(10, 2)
+  /// Total amount due (equals subtotal for now; extensible for taxes/discounts)
+  totalAmount   Decimal       @db.Decimal(10, 2)
+  /// Amount paid so far — transactional convenience field, NOT a standalone stored balance.
+  /// Always updated atomically with journal entries.
+  amountPaid    Decimal       @default(0) @db.Decimal(10, 2)
+  status        InvoiceStatus @default(DRAFT)
+  /// Journal entry created when this invoice was generated (DR AR, CR Revenue)
+  journalEntryId String?
+  /// Timestamp when invoice was sent to subscriber
+  issuedAt      DateTime?
+  /// Timestamp when invoice was fully paid
+  paidAt        DateTime?
+  /// Timestamp when invoice was voided
+  voidedAt      DateTime?
+  createdAt     DateTime      @default(now())
+  updatedAt     DateTime      @updatedAt
+
+  lines         InvoiceLine[]
+
+  /// Invoice numbers must be unique within a tenant
+  @@unique([tenantId, invoiceNumber])
+  /// Prevent duplicate invoices for same subscriber + period
+  @@unique([tenantId, subscriberId, periodStart])
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+  @@index([tenantId, status])
+  @@index([tenantId, subscriberId])
+  @@index([tenantId, dueDate])
+}
+
+/// A single line item on an Invoice (e.g., "50 Mbps Monthly Service — $49.99").
+model InvoiceLine {
+  id          String   @id @default(uuid())
+  tenantId    String
+  invoiceId   String
+  invoice     Invoice  @relation(fields: [invoiceId], references: [id])
+  description String
+  quantity    Int      @default(1)
+  unitPrice   Decimal  @db.Decimal(10, 2)
+  lineTotal   Decimal  @db.Decimal(10, 2)
+  createdAt   DateTime @default(now())
+
+  @@index([invoiceId])
+  @@index([tenantId])
+}