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

1. Extract parseTokenKey helper function to eliminate code duplication
2. Add channelId validation to prevent invalid formats like sk-abc:def:ghi
3. Add database migration to drop deprecated bound_channel_id column

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

- Add support for sk-{key}:{channelId} format to specify channel dynamically
- Remove BoundChannelId field and related code
- All users can now specify channels, not just admins
- Add model validation when channel is specified
- Remove bound channel UI from token management

Breaking change: tokens can no longer be bound to channels at creation time.
Use sk-{key}:{channelId} format instead.

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

When a token has BoundChannelId set, the distributor now uses that channel
instead of the normal channel selection process. The bound channel is still
validated to ensure it is enabled and supports the requested model.

Changes:
- Add ContextKeyTokenBoundChannelId constant
- Set bound channel in context during token authentication
- Check and use bound channel in distributor before normal selection

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