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>

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

删除了用户通过 sk-{key}:{channelId} 指定渠道、Playground 下拉框选择渠道、
以及管理员设置默认渠道 (is_default) 的所有后端代码。

涉及修改：
- middleware/auth.go: 简化 parseTokenKey，移除 specific_channel_id 设置
- middleware/distributor.go: 移除指定渠道分发逻辑、默认通道检查、ModelRequest.ChannelId
- constant/context_key.go: 移除 ContextKeyTokenSpecificChannelId
- controller/playground_channels.go: 整个文件删除
- controller/relay.go: 移除 shouldRetry/shouldRetryTaskRelay 中的指定渠道跳过
- controller/channel.go: 移除 GetUserChannelsForBinding
- controller/channel_pricing.go: 移除 SetDefaultChannel/ClearDefaultChannel
- model/ability.go: 移除 GetModelChannelsForGroup
- model/channel.go: 移除 GetAllChannelsForBinding
- model/channel_pricing.go: 移除默认通道缓存及所有相关函数
- model/pricing.go: 移除 DefaultChannelName 字段
- router/api-router.go: 移除 4 条路由
- i18n: 移除 channel.id_format_error 和 distributor.invalid_channel_id

为 Channel 模型新增 public_name 字段，让管理员可以为每个渠道
设置用户可见的友好名称（如"标准通道"、"高速通道"），替代前端
硬编码的"通道一/二/三"。Playground 通道选择器展示对外名称，
"渠道"统一改为"通道"。新建渠道时对外名称必填，编辑时可选。

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

Add a new /api/user/channels endpoint that returns only enabled channels
with non-sensitive fields (id, name, type, remark) for token channel binding.
Update EditTokenModal to use this endpoint and display remark instead of ID
in the channel dropdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>