feat(channel): 对齐 vivid 最新渠道/模型管理设计

渠道管理(对齐 9db9971/9f6eeca/b607a8d/268a8f7/7f8a87e):
- 列表新增「运行中」(Redis 并发槽位实时计数,与调度同源)和「失败数」
  (event_logs 按渠道聚合)两列
- 行内「同步上游模型」恢复弹窗:默认全开、右侧开关、彩点+单行别名,
  底部「同步模型」批量导入;编辑表单去掉获取模型列表按钮(保存已自动
  全量同步);去掉勾选即解绑语义与 unbind-model 端点
- 导入加固:别名与上游同名报错、同批别名复用拒绝、本地类型冲突拒绝、
  单次上限 50

模型管理(对齐 5545195):
- 模型表单/CreateModel 补齐 max_reference_videos/audios 与
  max_ref_video/audio_seconds 透传

产物(对齐 2253e58):
- 下载结果仅信任 image/*/video/* 响应头,text/plain 等乱报一律按魔数
  嗅探定 MIME

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

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

backend/internal/upstream/client.go

@@ -422,8 +422,13 @@ func (c *Client) Download(ctx context.Context, url string) ([]byte, string, erro
 	if len(data) == 0 {
 		return nil, "", fmt.Errorf("%w: empty result", ErrRetryable)
 	}
+	// Content-Type fallback chain: only trust image/* video/* headers (upstreams
+	// often mislabel results text/plain or octet-stream), else sniff magic bytes.
 	ct := strings.TrimSpace(resp.Header.Get("Content-Type"))
-	if ct == "" || ct == "application/octet-stream" {
+	if i := strings.IndexByte(ct, ';'); i >= 0 {
+		ct = strings.TrimSpace(ct[:i])
+	}
+	if !strings.HasPrefix(ct, "image/") && !strings.HasPrefix(ct, "video/") {
 		ct = http.DetectContentType(data)
 	}
 	return data, ct, nil