Log a storefront customer out

With a stateless session token, a plain logout is a client-side token discard the server simply acknowledges (204). `everywhere = true` bumps `customers.token_invalidated_at`, invalidating EVERY prior session for this customer (Q6). Behind the publishable key, the `customer_accounts` flag, and the customer-auth middleware (a valid session token is required). > **Note:** This endpoint requires the e-commerce feature to be enabled on your site.