diag: invalid_grant is SSO auth-method mismatch, not bad creds

- feat: creds-test.sh gum probe for SMTP vs PeerTube auth + PeerTubeService::probeAuth()
- feat: app-token.sh gum probe for long-lived PeerTube app token (client_credentials)
- docs: add copy-paste proof commands to demonstrate the SSO break to admins

.gitignore

@@ -58,6 +58,9 @@ app/public/assets/dist/
 # PHP CS Fixer cache
 .php-cs-fixer.cache
 
+# creds-test probe log (may contain error traces — no secrets, but generated per-run)
+creds-test.log
+
 # PHPUnit
 .phpunit.result.cache
 coverage/