Add BytePlus ModelArk as a new OpenAI-compatible provider for Seed 2.0
models (chat, vision). Two provider types: standard API and Coding Plan
(separate base URLs, same auth).
Integrate Seedream image generation (sync API) and Seedance video
generation (async polling) into the builtin media tool chain, following
the established DashScope/Gemini patterns.
- Add WithAuthPrefix option to OpenAIProvider for future non-standard auth
- Add ProviderBytePlus/ProviderBytePlusCoding store constants and config
- Register provider from config.json and llm_providers DB table
- Add BytePlus to media chain routing, priority lists, and dispatch
- Create create_image_byteplus.go (Seedream, sync response)
- Create create_video_byteplus.go (Seedance, async poll with 5min timeout)
- Add BytePlus to web and desktop UI provider type dropdowns
- Update provider docs with BytePlus entries
Closes#686
