- Add creatomateVideoGen.ts service with polling-based async rendering
- Replace video stub (setTimeout) with actual Creatomate API calls
- Add video preview (<video> tag) and MP4 download support
- Build programmatic source (branded slideshow) without pre-built templates
- Error handling: auth, rate limit, render failure → Korean messages
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>