Complete the DoubaoVideo asset library integration on top of the
managed per-user asset groups:

- Asset endpoint resolution is an explicit three-level priority: a
  hard-coded override wins, otherwise the per-channel credential
  base_url from channel_asset_credentials is used verbatim (it already
  contains the full /openApi/portrait path), otherwise the official
  default. The channel video base URL no longer participates in asset
  routing, so video (official Ark) and asset (gateway) addresses stay
  fully independent. The channel form gains an "素材 API 地址" field with
  a hint that the official address has no asset API, and the channel
  type label becomes "豆包视频（素材网关）" to surface the split. Credential
  summaries now cover DoubaoVideo channels and echo the base URL back
  for editing.

- After an asset request auto-matches a channel (no existing user
  binding), persist it to user_asset_channels so subsequent asset and
  video requests stay on the same channel, keeping asset:// references
  consistent with the upload channel. This mirrors the video-task
  binding backfill; binding failure logs a warning and does not fail
  the asset operation.

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

Bring official Volcengine channels in line with the China Mobile asset
isolation model: the platform owns the upstream asset group lifecycle
per (user, channel). asset_group.* APIs are forbidden for clients;
CreateAsset is scoped to the user's managed group (client GroupId is
overwritten), ListAssets is scoped with Filter.GroupIds to the managed
group, and Get/Update/Delete verify ownership via GetAsset before
forwarding, returning not-found for foreign assets.

Generalize the shared managed-group plumbing (GetOrCreateUserAssetGroup,
ScopeManagedAssetRequest, RequireManagedAssetOwnership) and add the
DoubaoVideo-specific group creator (Ark CreateAssetGroup returns the id
in Result.Id; GroupType must be omitted). The DoubaoVideo asset adapter
now serializes the request body from req.Body so platform rewrites take
effect (RawBody was bypassing the scoping) and the V4 signature always
covers the actual payload.

Verified end-to-end: group ops rejected, forged GroupId overwritten by
the auto-created managed group, ListAssets returns only the managed
group, foreign assets hidden from get/delete.

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

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>