feat(export): add video course export

[wangxiaoming1988]

Summary

• add MP4 video course export with selectable resolution
• include narration audio in ffmpeg composition and show detailed render errors
• cache generated narration audio by text and TTS settings to avoid duplicate synthesis

Checks

• pnpm exec tsc --noEmit
• pnpm exec eslint app/api/export/video-course/route.ts components/stage/header-controls.tsx lib/export/use-export-video-course.ts
• pnpm run check:i18n-keys
• pnpm run build

[danishsshaikh]

This is a feature I also need.

My use case is exporting the full generated classroom playback as a single video file, including:

• scene visuals
• narration/audio
• playback timing
• scene transitions where applicable

This would make it easier for teachers to share generated lessons outside the interactive OpenMAIC app, for example as an MP4 in an LMS or lecture archive.

I’m happy to help test this on self-hosted classrooms or pick up a focused follow-up if there is a smaller subtask that would be useful.