Core API v3 reference now matches live responses

We checked every v3 endpoint against the live API and corrected the reference.

Response fields now match what the API returns.

• Order fulfillments return id and order_id (previously documented as yotpo_id and yotpo_order_id).
• Products and variants return is_valid_url_format (previously is_valid_url).
• Order line items return product_id (previously yotpo_product_id).
• Newly documented: order_number, source, store_id, timestamps, pagination and webhook source.

Examples and required fields work as written.

• Fixed examples for Update order, Create and Update order fulfillment, Send aggregated order info, and Add and Remove product from collection.
• Corrected required fields for checkouts, customers, webhook filters and webhook subscriptions.
• Customers are identified by external_id or email. A customer with only phone_number is rejected.
• order_date_min and order_date_max need a full date and time.

Webhook payloads on Topics reflect real events.

• Loyalty payloads use customer.referred_by_customer (previously referred_by) and include website_url, store_name and published_at.
• Tier Status Changed uses snake_case vip_stats fields.

Also: every endpoint now documents its 401 and 403 responses, and the SMS & Email Analytics and Subscribers endpoints are marked deprecated.