feat(upstream): custom 模型按媒体分类配置参考文件上限

- ModelConfig 新增 max_reference_videos / max_reference_audios
  (配合既有 max_reference_images);admin 读写接口透传
- custom 模型表单去掉「参考模式」下拉,改为参考图/参考视频/
  参考音频三个数量输入;合计张数走 totalRefLimit 校验
- vivid 上游 /v1/uploads 按文件魔数自动选 kind(image/video/
  audio)与 mime;openai 协议上游遇视频/音频参考自动跳过换号
- videoCaps 让 custom 模型的分类上限在画图台/文档徽标生效
- reference_mode 请求级 override 放行 "media"

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

backend/internal/service/admin_read.go

@@ -102,6 +102,8 @@ func (s *AdminReadService) ModelsView(ctx context.Context) ([]map[string]any, er
 			"duration_prices_agent": map[string]any(item.DurationPricesAgent),
 			"durations":             repo.JSONStrings(item.Durations),
 			"max_reference_images":  item.MaxReferenceImages,
+			"max_reference_videos":  item.MaxReferenceVideos,
+			"max_reference_audios":  item.MaxReferenceAudios,
 			"reference_mode":        item.ReferenceMode,
 			"weight":                item.Weight,
 			"generation_count":      item.GenerationCount,