Support Microsoft Graph API for email delivery as alternative to SMTP AUTH

Summary Add support for email delivery via the Microsoft Graph API (OAuth 2.0 client credentials flow) as an alternative to SMTP with Basic Authentication. Problem Organizations using Microsoft 365 are increasingly unable to use Outline's SMTP email delivery because: Microsoft is actively deprecating Basic Authentication for SMTP Many M365 tenants have SMTP AUTH disabled by default as a security best practice Enabling per-mailbox SMTP AUTH requires opening a legacy authentication pathway that conflicts with Zero Trust and Conditional Access policies Users in these environments cannot send invite emails, document update notifications, or any other Outline-generated email This is a growing problem for higher education and enterprise M365 customers who manage tight security postures. Proposed Solution Add Microsoft Graph API as an optional email delivery provider alongside the existing SMTP implementation. The Graph API supports sending email via the POST /v1.0/users/{…