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/e2e_conversion_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
 
 	"nex/backend/internal/conversion"
 	"nex/backend/internal/conversion/anthropic"
@@ -48,10 +49,14 @@ func setupE2ETest(t *testing.T) (*gin.Engine, *httptest.Server) {
 	modelRepo := repository.NewModelRepository(db)
 	statsRepo := repository.NewStatsRepository(db)
 
-	providerService := service.NewProviderService(providerRepo, modelRepo)
-	modelService := service.NewModelService(modelRepo, providerRepo)
-	routingService := 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)
+	routingService := service.NewRoutingService(routingCache)
+	statsService := service.NewStatsService(statsRepo, statsBuffer)
 
 	registry := conversion.NewMemoryRegistry()
 	require.NoError(t, registry.Register(openaiConv.NewAdapter()))