Bunny Billing

Invoice, payment, and credit-note handling in Bunny. Covers the invoice state machine (DRAFT, PREPARING, READY, NOT_DUE, DUE, UNPAID, PROCESSING_PAYMENT, PAID, VOIDED, FAILED, MERGED, QUEUED, CONSOLIDATED), the credit-note lifecycle (DRAFT → READY → APPLIED / PARTIALLY_APPLIED), how invoices are generated by the cron vs `invoiceGenerateNextPeriod`, the void-vs-credit-vs-refund decision, the Payment + PaymentApplication model with multi-invoice application (`paymentApplyToInvoices`, `paymentApplyToAccount`), crediting (`invoiceCredit`, `invoiceAddCredit`, `creditNoteCreate` / `creditNoteIssue` / `creditNoteApplyToInvoices`), refund via `invoiceRefund`, consolidation billin…