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/conversion/openai/stream_encoder_test.go

@@ -27,8 +27,12 @@ func TestStreamEncoder_MessageStart(t *testing.T) {
 	data := strings.TrimPrefix(s, "data: ")
 	data = strings.TrimRight(data, "\n")
 	require.NoError(t, json.Unmarshal([]byte(data), &payload))
-	choices := payload["choices"].([]any)
-	delta := choices[0].(map[string]any)["delta"].(map[string]any)
+	choices, okch := payload["choices"].([]any)
+	require.True(t, okch)
+	msgMap, okmm := choices[0].(map[string]any)
+	require.True(t, okmm)
+	delta, okd := msgMap["delta"].(map[string]any)
+	require.True(t, okd)
 	assert.Equal(t, "assistant", delta["role"])
 }