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

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

backend/internal/upstream/client.go

@@ -95,6 +95,9 @@ type RemoteModel struct {
 	// Combined reference duration caps in seconds; 0 = unlimited.
 	RefVideoSeconds int64 `json:"refVideoSeconds,omitempty"`
 	RefAudioSeconds int64 `json:"refAudioSeconds,omitempty"`
+	// Bound is set by the admin fetch-models endpoint only: true when this
+	// upstream id is already bound to that channel locally.
+	Bound bool `json:"bound,omitempty"`
 }
 
 // RemoteDuration mirrors the duration spec of a vivid model item.