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/anthropic/adapter.go

@@ -141,7 +141,10 @@ func (a *Adapter) EncodeError(err *conversion.ConversionError) ([]byte, int) {
 			Message: err.Message,
 		},
 	}
-	body, _ := json.Marshal(errMsg)
+	body, marshalErr := json.Marshal(errMsg)
+	if marshalErr != nil {
+		return []byte(`{"type":"error","error":{"type":"internal_error","message":"internal error"}}`), statusCode
+	}
 	return body, statusCode
 }
 
@@ -235,7 +238,11 @@ func locateModelFieldInRequest(body []byte, ifaceType conversion.InterfaceType)
 			return "", nil, err
 		}
 		rewriteFunc := func(newModel string) ([]byte, error) {
-			m["model"], _ = json.Marshal(newModel)
+			encodedModel, err := json.Marshal(newModel)
+			if err != nil {
+				return nil, err
+			}
+			m["model"] = encodedModel
 			return json.Marshal(m)
 		}
 		return current, rewriteFunc, nil
@@ -269,7 +276,11 @@ func (a *Adapter) RewriteResponseModelName(body []byte, newModel string, ifaceTy
 	switch ifaceType {
 	case conversion.InterfaceTypeChat:
 		// Chat 响应必须有 model 字段，存在则改写，不存在则添加
-		m["model"], _ = json.Marshal(newModel)
+		encodedModel, err := json.Marshal(newModel)
+		if err != nil {
+			return nil, err
+		}
+		m["model"] = encodedModel
 		return json.Marshal(m)
 	default:
 		return body, nil