Start Payout Onboarding

Returns {url, account_id}. Worker visits the URL to onboard their bank account via Stripe Connect Express. Once they return, we'll know they've connected (Stripe webhook + GET /payouts updates the flag).

If the worker already has a stripe_account_id (e.g. an earlier attempt didn't finish onboarding), reuse that account and just issue a fresh AccountLink -- AccountLinks expire after a few minutes. Creating a new Account on every call would silently orphan whichever ones the worker didn't finish, and this row would end up pointing at the wrong one.