Redeem staking points for plan time or credits

Spends points from the wallet on the account.

quantity is months for pro_months and points for credits; the response always states the cost actually charged.

Pro months extend an existing period rather than replacing it, so redeeming twice does not discard the time already granted. They are refused when the organisation has a Stripe subscription, or is on a tier above Pro: a comped tier would overwrite the plan being paid for and would never expire. Credits apply on top of a subscription and are the right choice there.

Redemption serialises on the wallet's ledger, so two concurrent requests cannot spend the same points.