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/admin_service.go

@@ -575,18 +575,33 @@ func (s *adminServiceImpl) validateFallbackGroup(ctx context.Context, currentGro
 		return fmt.Errorf("cannot set self as fallback group")
 	}
 
-	// 检查降级分组是否存在
-	fallbackGroup, err := s.groupRepo.GetByID(ctx, fallbackGroupID)
-	if err != nil {
-		return fmt.Errorf("fallback group not found: %w", err)
-	}
+	visited := map[int64]struct{}{}
+	nextID := fallbackGroupID
+	for {
+		if _, seen := visited[nextID]; seen {
+			return fmt.Errorf("fallback group cycle detected")
+		}
+		visited[nextID] = struct{}{}
+		if currentGroupID > 0 && nextID == currentGroupID {
+			return fmt.Errorf("fallback group cycle detected")
+		}
 
-	// 降级分组不能启用 claude_code_only，否则会造成死循环
-	if fallbackGroup.ClaudeCodeOnly {
-		return fmt.Errorf("fallback group cannot have claude_code_only enabled")
-	}
+		// 检查降级分组是否存在
+		fallbackGroup, err := s.groupRepo.GetByIDLite(ctx, nextID)
+		if err != nil {
+			return fmt.Errorf("fallback group not found: %w", err)
+		}
 
-	return nil
+		// 降级分组不能启用 claude_code_only，否则会造成死循环
+		if nextID == fallbackGroupID && fallbackGroup.ClaudeCodeOnly {
+			return fmt.Errorf("fallback group cannot have claude_code_only enabled")
+		}
+
+		if fallbackGroup.FallbackGroupID == nil {
+			return nil
+		}
+		nextID = *fallbackGroup.FallbackGroupID
+	}
 }
 
 func (s *adminServiceImpl) UpdateGroup(ctx context.Context, id int64, input *UpdateGroupInput) (*Group, error) {