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/interface.go

@@ -4,10 +4,10 @@ package conversion
 type InterfaceType string
 
 const (
-	InterfaceTypeChat      InterfaceType = "CHAT"
-	InterfaceTypeModels    InterfaceType = "MODELS"
-	InterfaceTypeModelInfo InterfaceType = "MODEL_INFO"
-	InterfaceTypeEmbeddings InterfaceType = "EMBEDDINGS"
-	InterfaceTypeRerank    InterfaceType = "RERANK"
+	InterfaceTypeChat        InterfaceType = "CHAT"
+	InterfaceTypeModels      InterfaceType = "MODELS"
+	InterfaceTypeModelInfo   InterfaceType = "MODEL_INFO"
+	InterfaceTypeEmbeddings  InterfaceType = "EMBEDDINGS"
+	InterfaceTypeRerank      InterfaceType = "RERANK"
 	InterfaceTypePassthrough InterfaceType = "PASSTHROUGH"
 )