- 新增 is_default 字段和缓存层，支持管理员为模型指定默认通道
- Distribute 中间件优先级调整：Token 指定 → 默认通道 → 亲和性 → 随机
- 模型定价卡片和详情弹窗展示默认通道 amber 标识
- 管理后台定价页面新增星标切换默认通道
- 新增 set_default / clear_default API 和 6 个单元测试
- 简化卡片价格显示（移除内联缓存价格，改为详情弹窗展示）

Co-Authored-By: Claude <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>