Sync a Drip customer to Stripe (create cus_xxx and link)

Create a Stripe Customer on the merchant's connected Stripe account for the given Drip customer and persist the resulting cus_xxx on the Drip customer row. Idempotent: if the Drip customer already has a stripeCustomerId, returns it unchanged. Works with API-key or OAuth-connected Stripe integrations. Email/name are pulled from the Drip customer's metadata.email / metadata.name, falling back to externalCustomerId when it looks like an email, then to emailOverride from the request body.

Requires a secret key (sk_*) with at least the OPERATOR role.