feat: add messaging, webhooks, Xero and QuickBooks integrations

[Uyoxy]

Summary

This PR implements the following backend features:

• BE-39: In-app Messaging
• BE-40: Outbound Webhooks / Zapier Integration
• BE-41: Xero Accounting Integration
• BE-42: QuickBooks Online Accounting Integration

Changes

BE-39 – In-app Messaging

• Added MessagingModule
• Created MessageThread and Message entities
• Implemented thread creation and retrieval
• Added paginated message history endpoint
• Implemented sending messages within threads
• Updated thread lastMessageAt on new messages
• Added endpoint to mark thread messages as read
• Added unread message count endpoint
• Integrated real-time new-message WebSocket notifications

BE-40 – Outbound Webhooks

• Added WebhooksModule
• Created WebhookEndpoint entity
• Implemented webhook registration, listing, and deletion
• Added HTTPS URL validation
• Generated unique webhook secrets
• Integrated Bull queue for asynchronous webhook delivery
• Implemented HMAC SHA-256 request signing
• Added retry logic with exponential backoff
• Logged webhook delivery attempts

BE-41 – Xero Integration

• Added Xero integration module
• Implemented OAuth 2.0 PKCE authentication flow
• Added secure token storage
• Added connection status and disconnect endpoints
• Synced paid invoices and successful payments to Xero
• Added automatic token refresh
• Implemented nightly reconciliation job

BE-42 – QuickBooks Integration

• Added QuickBooks integration module
• Implemented OAuth authentication flow
• Added secure token storage
• Added connection status and disconnect endpoints
• Synced paid invoices and successful payments
• Implemented nightly reconciliation
• Added configuration support for QuickBooks environment variables

Testing

• Verified messaging CRUD flows
• Verified unread count and read status updates
• Verified WebSocket notifications for new messages
• Verified webhook registration and delivery flow
• Verified webhook retry behavior
• Verified Xero connection lifecycle
• Verified QuickBooks connection lifecycle
• Verified accounting sync triggers
• Verified scheduled reconciliation jobs

Notes

This PR introduces new backend modules and integrates them with the application module while maintaining existing functionality.

Closes

Closes #1225

Closes #1227

Closes #1228

Closes #1229

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