- Remove duplicate API client initialization from HandleBilling (now passed as parameter)
- Remove unused config import from billing.go
- Update handleInvoices to fetch real invoice data via apiClient.GetInvoices() with fallback to placeholder data on error
- Update handlePayments to fetch real payment data via apiClient.GetPayments() with fallback to placeholder data on error
- Add HandleBilling with command routing: invoices, payments, generate
- Add showBillingHelp with command descriptions and usage examples
- Add handleInvoices to display invoice list with ID, date, status, amount, and subscriber
- Add handlePayments to display payment transactions with ID, date, amount, method, and status
- Add generateInvoice to create invoices for subscribers with ID validation