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/tests/integration/integration_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
+	"go.uber.org/zap"
 	"gorm.io/gorm"
 
 	"nex/backend/internal/domain"
@@ -30,10 +31,14 @@ func setupIntegrationTest(t *testing.T) (*gin.Engine, *gorm.DB) {
 	modelRepo := repository.NewModelRepository(db)
 	statsRepo := repository.NewStatsRepository(db)
 
-	providerService := service.NewProviderService(providerRepo, modelRepo)
-	modelService := service.NewModelService(modelRepo, providerRepo)
-	_ = service.NewRoutingService(modelRepo, providerRepo)
-	statsService := service.NewStatsService(statsRepo)
+	logger := zap.NewNop()
+	routingCache := service.NewRoutingCache(modelRepo, providerRepo, logger)
+	statsBuffer := service.NewStatsBuffer(statsRepo, logger)
+
+	providerService := service.NewProviderService(providerRepo, modelRepo, routingCache)
+	modelService := service.NewModelService(modelRepo, providerRepo, routingCache)
+	_ = service.NewRoutingService(routingCache)
+	statsService := service.NewStatsService(statsRepo, statsBuffer)
 
 	providerHandler := handler.NewProviderHandler(providerService)
 	modelHandler := handler.NewModelHandler(modelService)