Add multi-session event series to attendee calendars now

Incoming registration webhooks without events leave attendees without session links and hosts lacking prep context. This creates session events on attendee calendars, giving hosts same-day schedules.

Overview

Registrations that arrive as webhooks often never become calendar events, leaving hosts without links and context. This flow turns each registration into a complete multi-session set on attendee calendars, giving community managers and program coordinators same-day schedule visibility and accurate session links.