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/tests/test_notify_dispatch.py

@@ -116,6 +116,33 @@ def test_notify_auth_login_bypasses_min_severity():
     mock_tg.notify_event.assert_called_once()
 
 
+def test_notify_rdg_connection_bypasses_min_severity():
+    event = Event(
+        event_id="00000000-0000-4000-8000-000000000603",
+        host_id=1,
+        occurred_at=datetime(2026, 5, 31, 18, 36, tzinfo=timezone.utc),
+        category="auth",
+        type="rdg.connection.success",
+        severity="info",
+        title="RD Gateway event 302",
+        summary="RDG 302 papatramp -> 192.168.160.3",
+        details={"telegram_via": "sac"},
+        payload={},
+    )
+    policy = NotificationPolicyConfig(
+        min_severity="warning",
+        use_telegram=True,
+        use_webhook=False,
+        use_email=False,
+        source="db",
+    )
+    with patch.object(notify_dispatch, "get_effective_notification_policy", return_value=policy):
+        with patch.object(notify_dispatch, "should_notify_event", return_value=True):
+            with patch.object(notify_dispatch, "telegram_notify") as mock_tg:
+                notify_dispatch.notify_rdg_connection(event)
+    mock_tg.notify_event.assert_called_once()
+
+
 def test_notify_auth_login_skips_telegram_when_via_agent():
     event = Event(
         event_id="00000000-0000-4000-8000-000000000602",