feat: 添加媒体播放功能，支持基于短时效token的播放入口及相关API接口

backend/app/http/tenant/dto/content.go

@@ -37,8 +37,8 @@ type ContentDetail struct {
 type ContentAssetsResponse struct {
 	// Content is the content entity.
 	Content *models.Content `json:"content,omitempty"`
-	// Assets is the list of media assets for the requested role (preview/main).
-	Assets []*models.MediaAsset `json:"assets,omitempty"`
+	// Assets is the list of playable assets for the requested role (preview/main).
+	Assets []*ContentPlayableAsset `json:"assets,omitempty"`
 	// PreviewSeconds indicates the max preview duration (only meaningful for preview response).
 	PreviewSeconds int32 `json:"preview_seconds,omitempty"`
 }