Website: add Playbooks / evaluations demo media

Fill the "Playbooks / evaluations" showcase row.

- **File:** `website/assets/showcase-eval.{mp4,gif,png,webp}`.
- **Where:** Showcase section, "Turn strategy into reusable checks".
- **Ratio:** 16:10.
- **Should show:** evaluation cards / checklist flagging points with evidence, and auto-checked to-dos as the AI detects they're addressed.

Part of #7.

