Stripe's customer portal for the clinic (§2.3).

400 when the clinic has no Stripe customer - ops-set and migrated clinics have none, and the account page hides the button for exactly that case (§3.3), the same contract the parent portal has at subscription/routes.py:883.

No fallback to Stripe's dashboard-default configuration. The default may allow the owner to edit her subscription's quantity, and seat quantity has exactly one writer (§5.2); a portal that can write it is a second one.