Convenience: send a test of the video-card CTA for a given course.

Body: to: single email address (required) course: slug from /courses (default lions-and-wolves). Available: lions-and-wolves, monkey-behind-the-wheel, petulant-child, teeter-totter, trading-as-combat subject: optional override; default uses the course title

Auto-detects whether the course's hero GIF is published. If yes, embeds it. If not, falls back to the SVG play-button card.