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/model_service_impl.go

@@ -0,0 +1,50 @@
+package service
+
+import (
+	appErrors "nex/backend/pkg/errors"
+
+	"nex/backend/internal/domain"
+	"nex/backend/internal/repository"
+)
+
+type modelService struct {
+	modelRepo    repository.ModelRepository
+	providerRepo repository.ProviderRepository
+}
+
+func NewModelService(modelRepo repository.ModelRepository, providerRepo repository.ProviderRepository) ModelService {
+	return &modelService{modelRepo: modelRepo, providerRepo: providerRepo}
+}
+
+func (s *modelService) Create(model *domain.Model) error {
+	// Verify provider exists
+	_, err := s.providerRepo.GetByID(model.ProviderID)
+	if err != nil {
+		return appErrors.ErrProviderNotFound
+	}
+	model.Enabled = true
+	return s.modelRepo.Create(model)
+}
+
+func (s *modelService) Get(id string) (*domain.Model, error) {
+	return s.modelRepo.GetByID(id)
+}
+
+func (s *modelService) List(providerID string) ([]domain.Model, error) {
+	return s.modelRepo.List(providerID)
+}
+
+func (s *modelService) Update(id string, updates map[string]interface{}) error {
+	// If updating provider_id, verify new provider exists
+	if providerID, ok := updates["provider_id"].(string); ok {
+		_, err := s.providerRepo.GetByID(providerID)
+		if err != nil {
+			return appErrors.ErrProviderNotFound
+		}
+	}
+	return s.modelRepo.Update(id, updates)
+}
+
+func (s *modelService) Delete(id string) error {
+	return s.modelRepo.Delete(id)
+}