Add Microsoft Outlook/Exchange CalDAV support

[PhilflowIO]

Enable migration from Microsoft Outlook and Exchange servers.

Requirements:

• OAuth2 support for Microsoft Graph API
• Handle Outlook-specific iCal properties
• Test with Office 365 and on-premise Exchange
• Document setup process (Azure AD app registration)

Technical Details:

• Microsoft uses OAuth2 (similar to Google)
• CalDAV endpoint: https://outlook.office365.com/EWS/Exchange.asmx/
• May need to handle Outlook-specific X-MS-* properties
• Rate limits: Microsoft Graph API quotas

Research Needed:

• Does tsdav support Microsoft's CalDAV implementation?
• Are there any known quirks or incompatibilities?

Estimated Effort: Large (5-7 days, including testing)

Related to: DAV Migration CLI - Phase 2