🚀 Feature: Support Microsoft Graph (OAuth2) as an outbound email provider

Problem

Plane's SMTP email integration (God Mode → Email settings) only supports basic SMTP authentication (host/port/username/password). Microsoft 365 tenants increasingly disable legacy/basic SMTP AUTH by default (Security Defaults / Conditional Access), which means self-hosted Plane instances using an Outlook/Microsoft 365 mailbox for outbound mail cannot authenticate at all — they hit 535 5.7.139 Authentication unsuccessful regardless of a correct password, and there is no way to send email through such a tenant today.

Proposal

Add Microsoft Graph API (/users/{mailbox}/sendMail) as an alternative email provider, authenticated via OAuth2 client-credentials (Azure AD app registration with Mail.Send application permission). This lets Plane send mail through Microsoft 365 mailboxes even when basic SMTP auth is disabled tenant-wide, without requiring any change to Plane's existing SMTP setup for everyone else.

Proposed approach:

• New InstanceConfiguration keys: EMAIL_PROVIDER (SMTP default, or MICROSOFT_GRAPH), EMAIL_GRAPH_TENANT_ID, EMAIL_GRAPH_CLIENT_ID, EMAIL_GRAPH_CLIENT_SECRET (encrypted) — same pattern as the existing SMTP keys, editable via the instance configuration API.
• A single EMAIL_BACKEND (PlaneEmailBackend) that transparently delegates to Django's existing SMTP backend unless EMAIL_PROVIDER is MICROSOFT_GRAPH, in which case it sends via Microsoft Graph's sendMail endpoint using a client-credentials token. This means every existing call site (magic link, invites, notifications, etc.) keeps working unchanged — nothing behaves differently unless an instance opts in.
• No new required dependency (requests is already a dependency).

I have a working implementation (backend module, Graph API client, and unit tests) and would like to submit it as a PR referencing this issue, per the contributing guide. Not proposing any admin UI changes in this first pass — configuration would be via the existing instance-configuration API/DB rows, same as SMTP is today. Happy to adjust scope based on maintainer feedback.

Start by tracing Plane's existing SMTP configuration through the instance-configuration API/DB rows and Django's email backend. Review the proposed backend module, Graph API client, and unit tests, then verify that SMTP remains the default, Microsoft Graph uses client-credentials configuration, and existing email call sites continue to work.