List the user's connections to MemoryFirst and TubExperto

The stored API key is never returned: only its last four characters, in secretHint. available is false when the server has no CONNECTIONS_SECRET_KEY, so connections cannot be stored or used. usage carries { used, limit } for a provider with a monthly cap (TubExperto, whose key is the user's own and spends their credits) and is null for one without (MemoryFirst).