refactor: 重构目录结构，后端代码统一到 backend/ 目录

backend/pkg/types/codes.go

@@ -0,0 +1,12 @@
+package types
+
+const (
+	CodeSuccess          = "0000"
+	InfoSuccess          = "success"
+	CodeUnknownError     = "0001"
+	InfoUnknownError     = "unknown error"
+	CodeIllegalParameter = "0002"
+	InfoIllegalParameter = "illegal parameter"
+	CodeAgentNotFound    = "0003"
+	InfoAgentNotFound    = "agent not found"
+)

backend/pkg/types/errors.go

@@ -0,0 +1,17 @@
+package types
+
+type AppError struct {
+	Code string
+	Info string
+}
+
+func NewAppError(code, info string) *AppError {
+	return &AppError{Code: code, Info: info}
+}
+
+func (e *AppError) Error() string {
+	if e == nil {
+		return ""
+	}
+	return e.Code + ": " + e.Info
+}