seedance-2-prompt — Seedance 2.0 video clip generator (dual backend)

Submits a Seedance 2.0 generation job and downloads the resulting MP4. Two backends share one CLI, picked via with.provider:

`with.provider`	Endpoint	Auth env	Default model
`openrouter` (default)	`https://openrouter.ai/api/v1/videos`	`OPENROUTER_API_KEY`	`bytedance/seedance-2.0`
`volcengine` (CN)	`https://ark.cn-beijing.volces.com/api/v3/contents/generations/tasks`	`ARK_API_KEY` (or `VOLC_ARK_API_KEY`)	`doubao-seedance-2-0-260128`
`byteplus` (intl)	`https://ark.ap-southeast.bytepluses.com/api/v3/contents/generations/tasks`	`ARK_API_KEY` (or `BYTEPLUS_API_KEY`)	`dreamina-seedance-2-0-260128`

Both flavours follow submit-then-poll, but differ in request shape (OpenRouter uses a flat prompt field; ARK packs everything into a content[] array), polling URL (OpenRouter returns polling_url; ARK gets id and you construct /contents/generations/tasks/{id}), the terminal-success status (completed vs succeeded), and where the final MP4 URL sits (top-level unsigned_urls[0] vs content.video_url). This script normalises both into a single Python contract.