feat: Store WhatsApp user_id in Contacts and include it in Webhook payloads to prevent duplicate profiles

Description Currently, when a contact interacts via WhatsApp, Chatwoot only stores the username. Because users can change their WhatsApp username at any time, this creates a major data integrity issue. If a user changes their username or hides their phone number, Chatwoot treats them as a completely new user, leading to duplicate contact profiles and fragmented conversation histories. Additionally, external automation tools (like n8n) connected via Chatwoot webhooks cannot reliably track users across username changes due to the lack of a permanent, unique identifier. To fix this, Chatwoot needs to fetch, store, and transmit the unique, immutable WhatsApp user_id. Desired Behavior • Data Storage: Fetch the unique, permanent WhatsApp user_id during webhook payload processing and save it to the Contact model. • Deduplication: Use this user_id as a primary lookup key alongside the phone number to match existing contacts before creating a new profile. • Webhook Integra…