Function next_invoice_number 

pub fn next_invoice_number(
    conn: &Connection,
    issuer: &Issuer,
    year: i32,
    kind: &str,
) -> Result<String>

Generate the next document number for an issuer/year/kind combination. kind is typically “invoice” or “credit_note” — credit notes get an independent sequence and are formatted with a “CN-” prefix.