refactor(settings): 简化流超时配置，移除冗余字段

- 移除 TimeoutSeconds 字段，超时判定由网关配置控制
- 默认禁用流超时处理功能

backend/internal/handler/dto/settings.go

@@ -70,7 +70,6 @@ type PublicSettings struct {
 // StreamTimeoutSettings 流超时处理配置 DTO
 type StreamTimeoutSettings struct {
 	Enabled                bool   `json:"enabled"`
-	TimeoutSeconds         int    `json:"timeout_seconds"`
 	Action                 string `json:"action"`
 	TempUnschedMinutes     int    `json:"temp_unsched_minutes"`
 	ThresholdCount         int    `json:"threshold_count"`