Record 60s demo showcase video for README #8

Context

README has an animated GIF; launch checklist also calls for a short MP4 showcase.

Scope

• Record 60s of /demo + dashboard on production or local with real providers
• Export to docs/assets/demo-showcase-60s.mp4
• Link from README and docs/demo/recording-checklist.md

References

• docs/demo/recording-checklist.md
• scripts/prove-real-voice.sh