refactor: generate a verifier on every oauth auth session

internal/service/google_oauth_service.go

@@ -48,10 +48,7 @@ func (google *GoogleOAuthService) Init() error {
 	httpClient := &http.Client{}
 	ctx := context.Background()
 	ctx = context.WithValue(ctx, oauth2.HTTPClient, httpClient)
-	verifier := oauth2.GenerateVerifier()
-
 	google.context = ctx
-	google.verifier = verifier
 	return nil
 }
 
@@ -65,6 +62,12 @@ func (oauth *GoogleOAuthService) GenerateState() string {
 	return state
 }
 
+func (google *GoogleOAuthService) GenerateVerifier() string {
+	verifier := oauth2.GenerateVerifier()
+	google.verifier = verifier
+	return verifier
+}
+
 func (google *GoogleOAuthService) GetAuthURL(state string) string {
 	return google.config.AuthCodeURL(state, oauth2.AccessTypeOffline, oauth2.S256ChallengeOption(google.verifier))
 }