refill credits at lunanode before expiry

i'm not sure we get a notification (even though we fixed the contact address in #40235 (closed)), so just make sure we don't run out of credit on lunanode.

the current dashboard says:

Sum: $8.20 monthly. You have $39.18 credits remaining (estimated credit exhaustion in 143.4 days).

so, let's set a warning in 110 days (~4 months?)