feat(cache): 实现 RoutingCache 和 StatsBuffer 优化数据库写入

- 新增 RoutingCache 组件，使用 sync.Map 缓存 Provider 和 Model
- 新增 StatsBuffer 组件，使用 sync.Map + atomic.Int64 缓冲统计数据
- 扩展 StatsRepository.BatchUpdate 支持批量增量更新
- 改造 RoutingService/StatsService/ProviderService/ModelService 集成缓存
- 更新 usage-statistics spec，新增 routing-cache 和 stats-buffer spec
- 新增单元测试覆盖缓存命中/失效/并发场景

backend/internal/service/routing_service_impl.go

@@ -4,20 +4,18 @@ import (
 	appErrors "nex/backend/pkg/errors"
 
 	"nex/backend/internal/domain"
-	"nex/backend/internal/repository"
 )
 
 type routingService struct {
-	modelRepo    repository.ModelRepository
-	providerRepo repository.ProviderRepository
+	cache *RoutingCache
 }
 
-func NewRoutingService(modelRepo repository.ModelRepository, providerRepo repository.ProviderRepository) RoutingService {
-	return &routingService{modelRepo: modelRepo, providerRepo: providerRepo}
+func NewRoutingService(cache *RoutingCache) RoutingService {
+	return &routingService{cache: cache}
 }
 
 func (s *routingService) RouteByModelName(providerID, modelName string) (*domain.RouteResult, error) {
-	model, err := s.modelRepo.FindByProviderAndModelName(providerID, modelName)
+	model, err := s.cache.GetModel(providerID, modelName)
 	if err != nil {
 		return nil, appErrors.ErrModelNotFound
 	}
@@ -26,7 +24,7 @@ func (s *routingService) RouteByModelName(providerID, modelName string) (*domain
 		return nil, appErrors.ErrModelDisabled
 	}
 
-	provider, err := s.providerRepo.GetByID(model.ProviderID)
+	provider, err := s.cache.GetProvider(model.ProviderID)
 	if err != nil {
 		return nil, appErrors.ErrProviderNotFound
 	}