Begin (or resume) Stripe Connect Express onboarding for THIS physician.

Two different kinds of idempotency, and the difference is the whole design:

• the ACCOUNT is created once, ever. A second account for the same physician is a second place their bank details could live and a payout destination that silently stops matching the one they onboarded.
• the LINK is minted fresh on every call, because account links are single-use and expire in minutes. Storing one would hand a physician a dead URL and no way to ask for another.

Nothing about the bank account comes back through here. Stripe hosts the form; we get an id and a redirect.