Receives push notifications from Google Calendar when watched events change. Used for RSVP tracking - when attendees accept/decline calendar invites, Google sends a notification here, and we update session_attendance records.

Setup Instructions:

1. Register your webhook URL with Google (see GOOGLE_CALENDAR_WEBHOOK.md)
2. Set GOOGLE_CALENDAR_WEBHOOK_TOKEN env var for verification
3. Webhook URL must be HTTPS in production

Headers from Google:

• X-Goog-Channel-ID: Your channel ID
• X-Goog-Resource-ID: Google's resource ID
• X-Goog-Resource-State: "sync", "exists", or "not_exists"
• X-Goog-Channel-Token: Your verification token
• X-Goog-Resource-URI: URI of changed resource