fix: show RD Gateway session duration in Telegram when present

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/telegram_templates.py

@@ -450,6 +450,9 @@ def _format_rdg_html(event: Event) -> str:
     err = _detail(details, "gateway_error_code", "error_code", default="")
     if err != "-":
         msg += _line("⚠️", "Код ошибки", html_escape(err))
+    dur = _detail(details, "session_duration_sec", default="")
+    if dur not in ("-", "0", ""):
+        msg += _line("⏱️", "Длительность", f"{html_escape(dur)} с")
     msg += _line("🕐", "Время", format_time(event.occurred_at))
     win_id = _detail(details, "event_id_windows", default="")
     if win_id != "-":