Retry sending an invoice

Retries delivery of an invoice whose previous send attempt failed. Safe pre-acceptance failures (delivery_status send_retryable) retry as-is. Provider-terminal failures (failed, send_failed) require a non-empty reason recorded in the audit trail — correct the recipient's e-invoice address and operator first; the resend creates a new e-invoice and the failed one stays on record. send_ambiguous sends are refused (409) until the outcome is verified with the operator. Requires a write-scoped key.