HeyGen AI video creation API. Use when: (1) Using Video Agent for one-shot prompt-to-video generation, (2) Generating AI avatar videos with /v2/video/generate, (3) Working with HeyGen avatars, voices, backgrounds, or captions, (4) Creating transparent WebM videos for compositing, (5) Polling video status or handling webhooks, (6) Integrating HeyGen with Remotion for programmatic video, (7) Translating or dubbing existing videos.