MembershipStripeSettingsController

Admin REST endpoints for the Settings → Membership → Payments panel. Ships in feature/membership-signup-checkout. Auth: manage_options + admin nonce. Provides GET and POST /membership/stripe for reading and saving Stripe keys and the test-mode toggle, plus POST /membership/stripe/test to verify the current keyset. Shares the same underlying StripeClient storage as ListenerSupportStripeSettingsController — saving keys through either panel is reflected immediately in both. Since feature/stripe-keys-reconciliation that is genuinely true in both directions; before it, this panel wrote the mode-specific keysets while the Listener Support panel wrote a separate mode-less slot, so propagation ran one way only. This panel also gained a webhook_secret field, which is why the older instruction to paste the signing secret into the Listener Support panel is obsolete.