feat: 配置 golangci-lint 静态分析并修复存量违规

- 新增 backend/.golangci.yml 配置 12 个 linter（forbidigo、errorlint、errcheck、staticcheck、revive、gocritic、gosec、bodyclose、noctx、nilerr、goimports、gocyclo）
- 新增 lefthook.yml 配置 pre-commit hook 自动运行 lint
- 修复存量代码违规：errors.Is/As 替换、zap.Error 替换、import 排序、errcheck 修复
- 更新 README 补充编码规范说明
- 归档 backend-code-lint 变更

backend/internal/service/model_service_impl.go

@@ -1,11 +1,15 @@
 package service
 
 import (
-	"github.com/google/uuid"
-	appErrors "nex/backend/pkg/errors"
+	"errors"
 
 	"nex/backend/internal/domain"
 	"nex/backend/internal/repository"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+
+	appErrors "nex/backend/pkg/errors"
 )
 
 type modelService struct {
@@ -108,7 +112,11 @@ func (s *modelService) Delete(id string) error {
 func (s *modelService) checkDuplicateModelName(providerID, modelName, excludeID string) error {
 	existing, err := s.modelRepo.FindByProviderAndModelName(providerID, modelName)
 	if err != nil {
-		return nil // 未找到，不重复
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil // 未找到，不重复
+		}
+
+		return err
 	}
 	if excludeID != "" && existing.ID == excludeID {
 		return nil // 排除自身