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_cache.go

@@ -0,0 +1,134 @@
+package service
+
+import (
+	"strings"
+	"sync"
+
+	"go.uber.org/zap"
+
+	"nex/backend/internal/domain"
+	"nex/backend/internal/repository"
+)
+
+type RoutingCache struct {
+	providers sync.Map
+	models    sync.Map
+
+	modelRepo    repository.ModelRepository
+	providerRepo repository.ProviderRepository
+	logger       *zap.Logger
+}
+
+func NewRoutingCache(
+	modelRepo repository.ModelRepository,
+	providerRepo repository.ProviderRepository,
+	logger *zap.Logger,
+) *RoutingCache {
+	return &RoutingCache{
+		modelRepo:    modelRepo,
+		providerRepo: providerRepo,
+		logger:       logger,
+	}
+}
+
+func (c *RoutingCache) GetProvider(id string) (*domain.Provider, error) {
+	if v, ok := c.providers.Load(id); ok {
+		return v.(*domain.Provider), nil
+	}
+
+	provider, err := c.providerRepo.GetByID(id)
+	if err != nil {
+		return nil, err
+	}
+
+	if v, ok := c.providers.Load(id); ok {
+		return v.(*domain.Provider), nil
+	}
+
+	c.providers.Store(id, provider)
+	return provider, nil
+}
+
+func (c *RoutingCache) GetModel(providerID, modelName string) (*domain.Model, error) {
+	key := providerID + "/" + modelName
+
+	if v, ok := c.models.Load(key); ok {
+		return v.(*domain.Model), nil
+	}
+
+	model, err := c.modelRepo.FindByProviderAndModelName(providerID, modelName)
+	if err != nil {
+		return nil, err
+	}
+
+	if v, ok := c.models.Load(key); ok {
+		return v.(*domain.Model), nil
+	}
+
+	c.models.Store(key, model)
+	return model, nil
+}
+
+func (c *RoutingCache) SetProvider(provider *domain.Provider) {
+	c.providers.Store(provider.ID, provider)
+}
+
+func (c *RoutingCache) SetModel(model *domain.Model) {
+	key := model.ProviderID + "/" + model.ModelName
+	c.models.Store(key, model)
+}
+
+func (c *RoutingCache) InvalidateProvider(id string) {
+	c.providers.Delete(id)
+	c.invalidateModelsByProvider(id)
+	c.logger.Debug("Provider 缓存失效", zap.String("provider_id", id))
+}
+
+func (c *RoutingCache) InvalidateModel(providerID, modelName string) {
+	key := providerID + "/" + modelName
+	c.models.Delete(key)
+	c.logger.Debug("Model 缓存失效",
+		zap.String("provider_id", providerID),
+		zap.String("model_name", modelName))
+}
+
+func (c *RoutingCache) invalidateModelsByProvider(providerID string) {
+	prefix := providerID + "/"
+	count := 0
+	c.models.Range(func(key, value interface{}) bool {
+		if strings.HasPrefix(key.(string), prefix) {
+			c.models.Delete(key)
+			count++
+		}
+		return true
+	})
+	if count > 0 {
+		c.logger.Debug("清除 Provider 相关 Model 缓存",
+			zap.String("provider_id", providerID),
+			zap.Int("count", count))
+	}
+}
+
+func (c *RoutingCache) Preload() error {
+	providers, err := c.providerRepo.List()
+	if err != nil {
+		return err
+	}
+	for i := range providers {
+		c.providers.Store(providers[i].ID, &providers[i])
+	}
+
+	models, err := c.modelRepo.List("")
+	if err != nil {
+		return err
+	}
+	for i := range models {
+		key := models[i].ProviderID + "/" + models[i].ModelName
+		c.models.Store(key, &models[i])
+	}
+
+	c.logger.Info("缓存预热完成",
+		zap.Int("providers", len(providers)),
+		zap.Int("models", len(models)))
+	return nil
+}