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/handler/handler_test.go

@@ -9,23 +9,22 @@ import (
 	"strings"
 	"testing"
 
+	"nex/backend/internal/domain"
+	"nex/backend/tests/mocks"
+
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/mock/gomock"
 	"gorm.io/gorm"
 
-	"nex/backend/internal/domain"
 	appErrors "nex/backend/pkg/errors"
-	"nex/backend/tests/mocks"
 )
 
 func init() {
 	gin.SetMode(gin.TestMode)
 }
 
-
-
 func TestProviderHandler_CreateProvider_MissingFields(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	defer ctrl.Finish()