Add /admin/invoices/stats with a year filter: invoiced, received and
outstanding totals in GEL at each invoice's captured rate (the one
currency every invoice has), overdue count, average days to pay, money
received per month by payment date as a bar chart with the same figures
in a table, per-currency totals, top clients and crypto option usage.
Cancelled invoices are left out of every amount; hidden ones follow the
usual superuser-only visibility.
Assisted-by: Claude:opus-5
