Allocate Commission Entry

PUT THE EARNING ON A BILL.

Writes the entry as a line item on the statement, which is what turns a recorded earning into something actually paid. The statement is scoped through the entry's OWN engagement, never an org id from the body, so an entry cannot be allocated onto someone else's statement.

Refused once the statement is locked for review or payment, and refused if the correction would take its total negative. Re-allocating to the same statement is idempotent; to a different one it is a 409.