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/routing_cache_test.go

@@ -5,11 +5,11 @@ import (
 	"sync"
 	"testing"
 
+	"nex/backend/internal/domain"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/zap"
-
-	"nex/backend/internal/domain"
 )
 
 type mockModelRepo struct {
@@ -189,7 +189,8 @@ func TestRoutingCache_InvalidateProvider_CascadingModels(t *testing.T) {
 
 	var openaiCount, anthropicCount int
 	cache.models.Range(func(key, value interface{}) bool {
-		if key.(string) == "anthropic/claude" {
+		keyStr, ok := key.(string)
+		if ok && keyStr == "anthropic/claude" {
 			anthropicCount++
 		}
 		return true