fix(docs): restore the ByteTrack demo video

docs/trackers/bytetrack.md pointed at a GitHub user-attachment that returns
a hard 404. It was never an expiry: cross-checking PR #401 (which added all
four tracker videos in 351fe5c) shows this UUID never appeared in that PR's
body, while a valid video uploaded in the same PR was referenced nowhere.
A UUID was transcribed incorrectly, so the page has shown a dead video since
2026-05-08.

Point it at the orphaned asset from that PR and add its poster. Confirmed
as the correct clip against the original ByteTrack render from the
2026-05-08 thread. The hosted asset is already h264 High, 1280x720, 25fps,
4.9 Mbps, 16.000s, matching its sibling tracker demos, so no re-upload or
re-encode is needed.

All 20 videos in the docs now have a poster.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: AlexBodner

docs/assets/bytetrack-demo-poster.webp

@@ -24,8 +24,8 @@ For comparisons with other trackers, plus dataset context and evaluation details
 
 ## Watch It in Action
 
-<video title="ByteTrack demo video" width="100%" style="aspect-ratio: 16/9;" controls preload="none">
-  <source src="https://github.com/user-attachments/assets/9c9736c9-ca78-48b3-b3c4-46d3e904dbb3" type="video/mp4">
+<video title="ByteTrack demo video" width="100%" style="aspect-ratio: 16/9;" controls preload="none" poster="../../assets/bytetrack-demo-poster.webp">
+  <source src="https://github.com/user-attachments/assets/7dfa0326-9b57-422c-899f-4729448bdded" type="video/mp4">
 </video>
 
 ## How does ByteTrack work?