Connect a Stripe account via API key

Validates Stripe credentials, creates a MarketplaceIntegration row (idempotent on Stripe account ID), optionally flips taxProvider to STRIPE_TAX, and returns the webhook URL the merchant must paste into their Stripe dashboard. Retrying this endpoint with the same Stripe account returns the existing integration with alreadyExists: true. Requires API_PUBLIC_BASE_URL to be set; webhook URL has no host header fallback. Requires a secret key with the ADMIN role.