Introduce an embeddable "on air" status widget. Adds Widgets::on_air controller action to lookup a user by callsign, sanitize input, and render a new widgets/on_air view (iframe-friendly). Adds Cat::recent_status_by_user_id to fetch recent CAT entries (last 15 minutes) for public display. View shows ON AIR/ QRT badges, radio/satellite details, and includes auto-refresh and minimal styling; usage: /widgets/on_air/YOURCALL.
