fix(分组): 防止降级环并校验上下文分组

- 增加降级链路环检测并拦截配置

- 仅复用合法分组上下文并必要时回退查询

- 标注 GetByIDLite 轻量语义并补充测试

backend/internal/service/group.go

@@ -72,3 +72,17 @@ func (g *Group) GetImagePrice(imageSize string) *float64 {
 		return g.ImagePrice2K
 	}
 }
+
+// IsGroupContextValid reports whether a group from context has the fields required for routing decisions.
+func IsGroupContextValid(group *Group) bool {
+	if group == nil {
+		return false
+	}
+	if group.ID <= 0 {
+		return false
+	}
+	if group.Platform == "" || group.Status == "" {
+		return false
+	}
+	return true
+}