Calendar invite generation via Resend

[SanAlexis]

Context

When a session is scheduled, auto-generate a calendar invite (.ics) via Resend containing:

• Session URL (ActiveBoard link)
• Meeting link (external Zoom/Teams/Meet URL from sessions.meeting_link)
• Date, time, and duration

Acceptance criteria

• Calendar invite generated and emailed to all group members when a session is scheduled
• Invite includes both the ActiveBoard session URL and the external meeting link
• Works with Resend free tier (3,000 emails/month)