Update a customer's contact and address details.

Send at least one field. Trying to move the customer to another merchant or tenant (merchantId, tenantId, merchant_id or tenant_id) returns 400 VALIDATION_ERROR. A customer of another merchant returns 404.

Changes apply to future payments. Payments already made keep their link to the customer.

Merchant users work on their own merchant's customers. Tenant owners and admins can work on any merchant in their tenant by passing ?merchantId=. A merchant in another tenant returns 404. Other roles are refused.

Authenticate with a dashboard session token, or a merchant-pinned secret key in X-API-Key. A secret key acts only for its own merchant: anything it creates belongs to that merchant and records the key as the actor, and another merchant's customers return 404. Publishable, payout_write and agent_onboarding keys are refused.