Set my cash position summary subscription

The signed-in member's own opt-in to the emailed cash-position summary for the business of their session. It is per member and off until the member turns it on; every recipient chooses for themselves. The summary goes to the member's address on file, at 5:30 PM Eastern on the due day: every day for daily, Mondays for weekly, the 1st of the month for monthly. Requires a signed-in business session and, on a business whose roles are enforced, the reports_read capability, because the summary states the business's balances; the send re-checks it, and a member who no longer holds it is not emailed. An API key is refused. The body carries the cadence only: any other field, such as an address or another member, is refused with a 400. A cadence other than daily, weekly, monthly or off is cash_position_summary_invalid_cadence (400); switching on with no address on file is cash_position_summary_no_member_email (400). Choosing what is already chosen records nothing.