Generate AI images using Google Vertex AI Imagen model

Start an AI video generation (Google Vertex AI Veo). Generation usually takes 1-3 minutes, so this returns immediately with an operationId. Do NOT poll right away: wait at least 60 seconds before the first get-video-status call, then poll every ~30 seconds until status is "done" to obtain the saved file path(s).

Check the status of a video generation started by generate-video. When status is "running", wait ~30 seconds before calling again. Stop when status is "done" (file paths are returned) or "error".