refactor(task): extract billing and polling logic from controller to service layer

Restructure the task relay system for better separation of concerns:
- Extract task billing into service/task_billing.go with unified settlement flow
- Move task polling loop from controller to service/task_polling.go (supports Suno + video platforms)
- Split RelayTask into fetch/submit paths with dedicated retry logic (taskSubmitWithRetry)
- Add TaskDto, TaskResponse generics, and FetchReq to dto/task.go
- Add taskcommon/helpers.go for shared task adaptor utilities
- Remove controller/task_video.go (logic consolidated into service layer)
- Update all task adaptors (ali, doubao, gemini, hailuo, jimeng, kling, sora, suno, vertex, vidu)
- Simplify frontend task logs to use new TaskDto response format

router/video-router.go

@@ -8,10 +8,16 @@ import (
 )
 
 func SetVideoRouter(router *gin.Engine) {
+	// Video proxy: accepts either session auth (dashboard) or token auth (API clients)
+	videoProxyRouter := router.Group("/v1")
+	videoProxyRouter.Use(middleware.TokenOrUserAuth())
+	{
+		videoProxyRouter.GET("/videos/:task_id/content", controller.VideoProxy)
+	}
+
 	videoV1Router := router.Group("/v1")
 	videoV1Router.Use(middleware.TokenAuth(), middleware.Distribute())
 	{
-		videoV1Router.GET("/videos/:task_id/content", controller.VideoProxy)
 		videoV1Router.POST("/video/generations", controller.RelayTask)
 		videoV1Router.GET("/video/generations/:task_id", controller.RelayTask)
 		videoV1Router.POST("/videos/:video_id/remix", controller.RelayTask)