- Add logrus import to invoice.go, subscriber_crud.go, subscriber_esim.go, and subscriber_lifecycle.go
- Replace fmt.Printf with logrus.WithError and WithField for structured error logging
- Update invoice email notification placeholder to use logrus.Info with invoice_id field
- Replace eSIM provisioning error print with logrus.Warn including subscriber_id field
- Add comment explaining derived
- Add InvoiceService with database dependency for invoice management operations
- Add GenerateMonthlyInvoice to create invoices with plan fees and usage charges from billing period
- Add SendInvoice to update invoice status to pending (email integration TODO)
- Add MarkInvoiceAsPaid to update invoice payment status
- Add GetOverdueInvoices to query invoices past due date with pending status
- Add GetSubscriber