- Fix shouldBillToday() month-wrapping logic for PREPAID lead days
- POST /api/billing/generate — triggers monthly invoice generation cycle
- GET /api/invoices — paginated list with status/subscriber/date filters
- GET /api/invoices/[id] — invoice detail with lines and subscriber
- POST /api/invoices/[id]/void — void with JE reversal
Test coverage (38 tests):
- computeBillingPeriod pure function
- shouldBillToday: postpaid, prepaid, and month-wrapping edge case
- Invoice number sequencing per tenant/year
- Invoice generation: amounts, InvoiceLine, period dates
- Journal entries: DR AR (1100), CR Revenue (4010), balanced
- Idempotency: duplicate prevention via unique(subscriberId, periodStart)
- Credit auto-application: full, partial, zero credit, JE (DR 1150, CR 1100)
- Billing cycle: active-only, suspended/cancelled excluded, prepaid lead days
- Overdue detection: bulk update of DRAFT/SENT/PARTIAL past due date
- Void: JE reversal, already-voided guard, PAID guard
- getInvoice, listInvoices, status filtering
- Tenant isolation: Tenant B cannot see Tenant A invoices
