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/repository/provider_repo_impl.go

@@ -3,10 +3,11 @@ package repository
 import (
 	"time"
 
-	"gorm.io/gorm"
-
 	"nex/backend/internal/config"
 	"nex/backend/internal/domain"
+
+	"gorm.io/gorm"
+
 	appErrors "nex/backend/pkg/errors"
 )
 

backend/internal/repository/repository_test.go

@@ -3,13 +3,13 @@ package repository
 import (
 	"testing"
 
+	"nex/backend/internal/domain"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
 
 	testHelpers "nex/backend/tests"
-
-	"nex/backend/internal/domain"
 )
 
 func setupTestDB(t *testing.T) *gorm.DB {

backend/internal/repository/stats_repo_impl.go

@@ -3,11 +3,11 @@ package repository
 import (
 	"time"
 
-	"gorm.io/gorm"
-	"gorm.io/gorm/clause"
-
 	"nex/backend/internal/config"
 	"nex/backend/internal/domain"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 )
 
 type statsRepository struct {
@@ -19,8 +19,8 @@ func NewStatsRepository(db *gorm.DB) StatsRepository {
 }
 
 func (r *statsRepository) Record(providerID, modelName string) error {
-	today := time.Now().Format("2006-01-02")
-	todayTime, _ := time.Parse("2006-01-02", today)
+	now := time.Now()
+	todayTime := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, now.Location())
 
 	stats := config.UsageStats{
 		ProviderID:   providerID,