Register a DoubaoVideo asset adapter that forwards the Ark-compatible
asset API (POST /api/v1/volcengine/asset?Action=xx) to the official
/openApi/portrait endpoint with a Volcengine V4 HMAC-SHA256 signature
(canonical request over content-type/host/x-content-sha256/x-date,
date/cn-beijing/ark/request scope). The AK/SK pair and optional
project code are stored in channel_asset_credentials (pool_id holds
the project code), so asset credentials stay separate from the video
Bearer key; the channel form now accepts them for type 54.

DoubaoVideo joins the seedance asset family so a user's asset upload
channel and video task channel stay the same (asset:// references
pass through to the upstream). Both resolution chains (asset proxy
resolver and video asset family matcher) now require the asset
credential to be configured before a type-54 channel can serve,
bind, or be auto-matched for assets, falling through to compatible
channels otherwise. The /api/v3 native-path whitelist simplifies
back to the family list now that 54 is a member.

Verified end-to-end against the real upstream: CreateAssetGroup,
CreateAsset (real video upload, Processing -> Active moderation),
ListAssets/GetAsset, and a video generation task referencing the
asset via asset://video reference (succeeded, 432900 tokens).

Co-Authored-By: ZCode <noreply@anthropic.com>