添加视频帧提取功能和阿里云OSS存储支持

- 新增从视频素材提取首帧/尾帧的功能，支持画面连续性编辑
- 添加阿里云OSS存储支持，可配置本地或OSS存储方式
- 导入视频素材时自动探测并更新视频时长信息
- 前端添加从素材提取尾帧的UI界面
- 添加FramePrompt模型的数据库迁移

Co-Authored-By: Claude <noreply@anthropic.com>

api/routes/routes.go

@@ -180,6 +180,7 @@ func SetupRouter(cfg *config.Config, db *gorm.DB, log *logger.Logger, localStora
 			assets.DELETE("/:id", assetHandler.DeleteAsset)
 			assets.POST("/import/image/:image_gen_id", assetHandler.ImportFromImageGen)
 			assets.POST("/import/video/:video_gen_id", assetHandler.ImportFromVideoGen)
+			assets.POST("/:id/extract-frame", assetHandler.ExtractFrame)
 		}
 
 		storyboards := api.Group("/storyboards")