gatus: watch the 99stack runway

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>

ansible/roles/gatus_endpoint/templates/endpoints.yaml.j2

@@ -41,6 +41,10 @@ endpoints:
 {% if e.client is defined %}
     client:
 {{ e.client | to_nice_yaml(indent=2) | indent(6, true) }}
+{% endif %}
+{% if e.headers is defined %}
+    headers:
+{{ e.headers | to_nice_yaml(indent=2) | indent(6, true) }}
 {% endif %}
     conditions:
 {% for c in e.conditions %}