List Client Activity

Each client's relationship in money: owed now, invoiced and collected in year, last invoice, average days to pay.

A separate read rather than fields on ClientRead, because ClientRead is also what the edit form round-trips: a figure there would be something a PUT could appear to write. Declared before /clients/{client_id} so the literal segment is not parsed as an id.

year defaults to the server's current year, as /incassi does. A client with no issued document has no entry — the table renders its absence as "nothing yet", not as a row of zeros.