Returns a single event. The response includes primary_event_id (always resolves to the original event ID, even for co-hosted mirrors) and is_co_hosted_mirror (true if this event is a mirror copy from a co-host relationship). Event sessions include primary_session_id for the same purpose. If the event has an event page, the response also includes image_url (the event page image), description (plain text content from the event page), and accessibility_info (an optional per-language hash of accessibility details from the event page settings, e.g. {"en": "Wheelchair accessible entrance"}; values may contain limited inline HTML: p, br, strong, em, a). These fields are null when no event page exists or the value is not set. An event that has no sessions returns 404.