QuickBooks OAuth Connection

Implement OAuth 2.0 authentication for QuickBooks Online with automatic token refresh to enable secure API access.

Note: - Use QuickBooks OAuth 2.0 flow (authorization code grant) - Store access token and refresh token securely in database - Implement automatic token refresh (tokens expire after 1 hour, refresh valid for 100 days) - Store realm ID (company ID) with connection - Support sandbox and production QuickBooks environments - Handle token refresh failures and require re-authentication - Store connection per customer (one QB connection per customer) - Reference PoC implementation for OAuth flow patterns

Open questions: - Where in Astrata Accounting should OAuth callback be handled? - How should connection status be displayed to admin users?