Receives push notifications from Google Calendar when watched events change. Used for RSVP tracking - when attendees accept/decline invites, Google sends a notification here, and we fetch updated attendee responses.

Note: Google sends headers, not body data. The headers include:

• X-Goog-Channel-ID: Your channel ID
• X-Goog-Resource-ID: Google's resource ID
• X-Goog-Resource-State: "sync" (initial), "exists" (change), or "not_exists" (deleted)
• X-Goog-Channel-Token: Your verification token (if set)