🔒️(backend) derive connection-test room max age from token TTL

Refactor CONNECTION_TEST_ROOM_MAX_AGE_SECONDS so it is no longer an
independent setting but a quantity derived from (or added on top of)
the token TTL.

This prevents a misconfiguration where the token would outlive the
delete-room callback. In that case, an attacker holding a valid
token could recreate the room after the callback fired and escape
the intended cleanup.

src/backend/core/api/viewsets.py

@@ -1612,9 +1612,13 @@ class DiagnosticsViewSet(viewsets.ViewSet):
         # eject someone who stays connected. Schedule a hard DeleteRoom when Celery
         # is available.
         if settings.CELERY_ENABLED:
+            max_age = (
+                settings.CONNECTION_TEST_TOKEN_TTL_SECONDS
+                + settings.CONNECTION_TEST_ROOM_EXTRA_AGE_SECONDS
+            )
             delete_connection_test_room.apply_async(
                 args=[room],
-                countdown=settings.CONNECTION_TEST_ROOM_MAX_AGE_SECONDS,
+                countdown=max_age,
             )
 
         return drf_response.Response(