feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/internal/service/routing_service_impl.go

@@ -0,0 +1,42 @@
+package service
+
+import (
+	appErrors "nex/backend/pkg/errors"
+
+	"nex/backend/internal/domain"
+	"nex/backend/internal/repository"
+)
+
+type routingService struct {
+	modelRepo    repository.ModelRepository
+	providerRepo repository.ProviderRepository
+}
+
+func NewRoutingService(modelRepo repository.ModelRepository, providerRepo repository.ProviderRepository) RoutingService {
+	return &routingService{modelRepo: modelRepo, providerRepo: providerRepo}
+}
+
+func (s *routingService) Route(modelName string) (*domain.RouteResult, error) {
+	model, err := s.modelRepo.GetByModelName(modelName)
+	if err != nil {
+		return nil, appErrors.ErrModelNotFound
+	}
+
+	if !model.Enabled {
+		return nil, appErrors.ErrModelDisabled
+	}
+
+	provider, err := s.providerRepo.GetByID(model.ProviderID)
+	if err != nil {
+		return nil, appErrors.ErrProviderNotFound
+	}
+
+	if !provider.Enabled {
+		return nil, appErrors.ErrProviderDisabled
+	}
+
+	return &domain.RouteResult{
+		Provider: provider,
+		Model:    model,
+	}, nil
+}