Add sender_name (WhatsApp display/push name) to the message object

Currently the message object identifies the sender only by phone-based IDs — author, from, and sender_phone all return values in the {phone_number}@c.us format. There's no field for the sender's WhatsApp display name (push name), which is what WhatsApp itself shows in group chats.

This means any consumer of the API has to either show raw phone numbers or maintain a separate phone → name mapping and keep it in sync via the contacts endpoint. That's an extra lookup per message.

Requested change: add a sender_name field to the message object, populated with the sender's WhatsApp push name (nullable when unavailable). Ideally this is also included in webhook payloads so real-time consumers don't need a follow-up API call.