feat: notify RD Gateway events in Telegram outside min_severity

Route rdg.connection.* through notify_rdg_connection and add disconnected type.

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

backend/app/services/telegram_templates.py

@@ -428,8 +428,12 @@ def format_event_telegram_html(event: Event) -> str:
 
 def _format_rdg_html(event: Event) -> str:
     details = _details_dict(event)
-    ok = event.type.endswith(".success")
-    header = "✅ RD Gateway: подключение" if ok else "❌ RD Gateway: ошибка"
+    if event.type.endswith(".success"):
+        header = "✅ RD Gateway: подключение"
+    elif event.type.endswith(".disconnected"):
+        header = "ℹ️ RD Gateway: отключение"
+    else:
+        header = "❌ RD Gateway: ошибка"
     msg = f"<b>{header}</b>\n"
     msg += _line("👤", "Пользователь", html_escape(_detail(details, "user", "username")))
     msg += _line("🏢", "Хост", host_label(event.host))