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/proxy_handler_test.go

@@ -8,27 +8,26 @@ import (
 	"net/http/httptest"
 	"testing"
 
+	"nex/backend/internal/conversion"
+	"nex/backend/internal/conversion/anthropic"
+	"nex/backend/internal/conversion/openai"
+	"nex/backend/internal/domain"
+	"nex/backend/internal/provider"
+	"nex/backend/tests/mocks"
+
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/mock/gomock"
 	"go.uber.org/zap"
 
-	"nex/backend/internal/conversion"
-	"nex/backend/internal/conversion/anthropic"
-	"nex/backend/internal/conversion/openai"
-	"nex/backend/internal/domain"
-	"nex/backend/internal/provider"
 	appErrors "nex/backend/pkg/errors"
-	"nex/backend/tests/mocks"
 )
 
 func init() {
 	gin.SetMode(gin.TestMode)
 }
 
-
-
 func setupProxyEngine(t *testing.T) *conversion.ConversionEngine {
 	t.Helper()
 	registry := conversion.NewMemoryRegistry()
@@ -844,7 +843,8 @@ func TestProxyHandler_HandleProxy_Models_LocalAggregation(t *testing.T) {
 	require.True(t, ok)
 	assert.Len(t, data, 2)
 
-	first := data[0].(map[string]interface{})
+	first, ok2 := data[0].(map[string]interface{})
+	require.True(t, ok2)
 	assert.Equal(t, "openai/gpt-4", first["id"])
 }
 
@@ -918,7 +918,7 @@ func TestProxyHandler_HandleProxy_SmartPassthrough_UnifiedID(t *testing.T) {
 	client := mocks.NewMockProviderClient(ctrl)
 	client.EXPECT().Send(gomock.Any(), gomock.Any()).DoAndReturn(func(ctx context.Context, spec conversion.HTTPRequestSpec) (*conversion.HTTPResponseSpec, error) {
 		var req map[string]interface{}
-		json.Unmarshal(spec.Body, &req)
+		require.NoError(t, json.Unmarshal(spec.Body, &req))
 		assert.Equal(t, "gpt-4", req["model"])
 
 		return &conversion.HTTPResponseSpec{