New "Test senden" button next to Speichern posts current form
credentials (not DB) to /actions/notifications/test, which fires a
test message and redirects back with a flash chip showing the outcome
(including the Telegram API's error description on failure).
telegram_send is now public and returns (ok, detail) so the UI can
surface real error messages ("chat not found", "Unauthorized", etc.).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>