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>