Onboard Payouts

Initiate Stripe Connect Express onboarding for a producer organization or enrollee.

SWEEP FIX: this mints/updates a Stripe Connect account and — once account_id comes back — WRITES it as the org's/enrollee's stripe_connect_account_id (the payout destination) and returns an onboarding URL that lets whoever holds it attach their OWN bank details as that destination. Auth used to be optional (skipped entirely with no Authorization header) and even when a token WAS presented, an org_id/enrollee_id named in the body was resolved and acted on with NO check that the caller's principal actually owned it — an anonymous or unrelated caller could redirect another producer's payouts. A verified producer token is now required, and an explicit org_id/ enrollee_id must be one the token's principal holds a Membership in.