Record that the owner downloaded this video, for the chargeback trail.

<br/>The file itself is fetched straight from the CDN by the browser, so the<br/>server never sees the transfer: this beacon is the only seam where "the<br/>customer took the finished video away" becomes a fact we can cite in a<br/>dispute. Ownership is enforced like every other video route, so nobody can<br/>stamp a download onto somebody else's trail.<br/><br/>It is also where video_downloaded fires, attributed to the CALLER (an<br/>admin reaching another tenant's video is who downloaded it). Full rationale:<br/>docs/ANALYTICS_EVENTS.md.