refactor: generate a verifier on every oauth auth session

internal/service/github_oauth_service.go

@@ -53,10 +53,7 @@ func (github *GithubOAuthService) Init() error {
 	httpClient := &http.Client{}
 	ctx := context.Background()
 	ctx = context.WithValue(ctx, oauth2.HTTPClient, httpClient)
-	verifier := oauth2.GenerateVerifier()
-
 	github.context = ctx
-	github.verifier = verifier
 	return nil
 }
 
@@ -70,6 +67,12 @@ func (github *GithubOAuthService) GenerateState() string {
 	return state
 }
 
+func (github *GithubOAuthService) GenerateVerifier() string {
+	verifier := oauth2.GenerateVerifier()
+	github.verifier = verifier
+	return verifier
+}
+
 func (github *GithubOAuthService) GetAuthURL(state string) string {
 	return github.config.AuthCodeURL(state, oauth2.AccessTypeOffline, oauth2.S256ChallengeOption(github.verifier))
 }