99stack is prepaid: when the balance runs out the servers stop. A pulled
check on the account API alerts when billing.secondsdue - the provider's own
runway at current spend - drops under 14 days: checked every 6h, reminding
every 24h. gatus_endpoint gains a `headers` field for APIs behind a
credential.
The vault gains ninetyninestack_api_token (account-wide; 99stack has no
read-only scope) and gatus_push_tokens.arbret, used by the next commit.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>