Website: add Live transcript demo media

Fill the "Live transcript" showcase row.

- **File:** `website/assets/showcase-transcript.{mp4,gif,png,webp}`.
- **Where:** Showcase section, "A clean record before analysis".
- **Ratio:** 16:10.
- **Should show:** speaker-separated transcript updating live, editable speaker names. A short clip of lines streaming in works best.

Part of #7.

