fix: module id and linting issues

pkg/model/error_test.go

@@ -3,8 +3,8 @@ package model
 import (
 	"errors"
 	"fmt"
-	"testing"
 	"net/http"
+	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"gopkg.in/yaml.v2"

pkg/model/model.go

@@ -16,6 +16,10 @@ type Subscriber struct {
 	Domain       string `json:"domain"`
 }
 
+type ContextKey string
+
+const ContextKeyModuleId ContextKey = "module_id"
+
 // Subscription represents subscription details of a network participant.
 type Subscription struct {
 	Subscriber       `json:",inline"`