feat(channels): 拉模型改为勾选即绑定,启动清扫遗留模型

- fetch-models 返回 bound 标记,渠道页勾选=立即导入绑定,
  取消=解绑(无其他渠道接入时本地模型行一并删除)
- 新增 POST /channels/unbind-model
- 启动时 PurgeOrphanModels 清掉 provider 时代的无绑定模型行
- 模型管理页移除「渠道导入模型」按钮

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/internal/http/router/router.go

@@ -147,6 +147,7 @@ func New(cfg *config.Config, auth *service.AuthService, handlers Handlers) *gin.
 		authed.POST("/channels/delete", handlers.ChannelAdmin.Delete)
 		authed.POST("/channels/fetch-models", handlers.ChannelAdmin.FetchModels)
 		authed.POST("/channels/import-models", handlers.ChannelAdmin.ImportModels)
+		authed.POST("/channels/unbind-model", handlers.ChannelAdmin.UnbindModel)
 		authed.GET("/providers", handlers.AdminRead.Providers)
 		authed.GET("/images", handlers.AdminRead.Images)
 		authed.DELETE("/images", handlers.AdminRead.DeleteImage)