Set or end a partner's co-marketing agreement

Record what this partner is paid per month, or end the agreement.

An amount lands as an agreement TERM, not a scalar. Changing the rate
opens a new term from today and closes the previous one — it does not
restate what earlier periods cost. That is what keeps a past quarter's
blended cost per funded loan stable after a rate change.

Send endAgreement: true to close the current term without starting a
new one. The response carries the RECOMPUTED cost figures, so a client
never has to re-request to refresh what it just changed.