fix(分组): 强化上下文分组可信校验

- 引入 Hydrated 标记限制复用来源

- 无效上下文分组允许被新值覆盖自愈

- 更新相关单测覆盖

backend/internal/service/group.go

@@ -10,6 +10,7 @@ type Group struct {
 	RateMultiplier float64
 	IsExclusive    bool
 	Status         string
+	Hydrated       bool // indicates the group was loaded from a trusted repository source
 
 	SubscriptionType    string
 	DailyLimitUSD       *float64
@@ -81,6 +82,9 @@ func IsGroupContextValid(group *Group) bool {
 	if group.ID <= 0 {
 		return false
 	}
+	if !group.Hydrated {
+		return false
+	}
 	if group.Platform == "" || group.Status == "" {
 		return false
 	}