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_supplemental_test.go

@@ -6,13 +6,13 @@ import (
 	"net/http/httptest"
 	"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"
-
-	"nex/backend/internal/domain"
-	"nex/backend/tests/mocks"
 )
 
 func TestProviderHandler_CreateProvider_Success(t *testing.T) {
@@ -24,9 +24,9 @@ func TestProviderHandler_CreateProvider_Success(t *testing.T) {
 	h := NewProviderHandler(mockSvc)
 
 	body, _ := json.Marshal(map[string]string{
-		"id":      "p1",
-		"name":    "Test",
-		"api_key": "sk-test",
+		"id":       "p1",
+		"name":     "Test",
+		"api_key":  "sk-test",
 		"base_url": "https://api.test.com",
 	})
 	w := httptest.NewRecorder()