- OverdueRenewalsAlert now shows participants with unpaid renewal invoices
  (not just those missing invoices), with amounts and program names
- EnrollmentService gates proration behind isEnabled() check
- calculateFirstBillingDate defaults to monthly day-1 when billing_cycle is NULL
- Alert shows total amount owed and links each participant to payment wizard
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>