SMSTunnel for Gatus

Help me get Gatus alerts as text messages from my own phone, with SMSTunnel.

About SMSTunnel (https://smstunnel.io): my own apps and tools hand text messages (SMS) to my SMSTunnel account, and my own Android phone sends them from my own number, on my own SIM and plan. The phone runs the SMSTunnel app from Google Play (https://play.google.com/store/apps/details?id=io.smstunnel.app), signed in to my account.

What I want: [describe what should send a text, to whom, and when]

How it works:
- No plugin: Gatus's "custom" alerting provider calls SMSTunnel, and each endpoint gets an alert of type custom. Two ways: a webhook address (no key; it texts up to 10 numbers saved with it) or SMSTunnel's API with a key.
- Webhook address way, config.yaml (the address made in my dashboard with Gatus picked; keep the Content-Type line, Gatus sends none of its own):
alerting:
  custom:
    url: "https://smstunnel.io/api/v1/webhook/<token>"
    method: "POST"
    headers:
      Content-Type: "application/json"
    body: |
      {"message": "[ALERT_TRIGGERED_OR_RESOLVED]: [ENDPOINT_NAME] - [ALERT_DESCRIPTION]"}

endpoints:
  - name: shop
    url: "https://shop.example.com/health"
    interval: 1m
    conditions:
      - "[STATUS] == 200"
    alerts:
      - type: custom
        failure-threshold: 3
        send-on-resolved: true
        description: "shop is down"
- Without a key: an SMSTunnel webhook address, https://smstunnel.io/api/v1/webhook/<token>. I make it in my dashboard (https://dashboard.smstunnel.io): Apps → Webhooks → New webhook, pick the tool that will call it (Grafana, Uptime Kuma, Healthchecks, Gatus, Alertmanager, Odoo Online, or Something else), add 1 to 10 numbers (each gets its own text), optionally a text of the message that pulls fields out of the JSON with {{path}}, like {{alarm.name}}, then Create the webhook and copy the address. A GET with ?message=… or a POST with JSON (its message, text, title or body field) texts those numbers; the call never sets the number. Texts are cut at 480 characters. "Calls an hour" (20 unless I set another number, up to 500) counts calls, not texts: each call texts every number, and calls past it are refused until the hour is over. The address can be copied again under Address and setup; New address replaces it if it leaks. Scripts can make one with POST https://smstunnel.io/api/v1/webhooks and my sign-in token instead.
- API key way, config.yaml:
alerting:
  custom:
    url: "https://smstunnel.io/api/v1/sms/send"
    method: "POST"
    headers:
      Content-Type: "application/json"
      X-API-Key: "${SMSTUNNEL_API_KEY}"
    body: |
      {
        "to": "+14155550123",
        "message": "[ALERT_TRIGGERED_OR_RESOLVED]: [ENDPOINT_NAME] - [ALERT_DESCRIPTION] - [RESULT_ERRORS]"
      }

endpoints:
  - name: shop
    url: "https://shop.example.com/health"
    interval: 1m
    conditions:
      - "[STATUS] == 200"
    alerts:
      - type: custom
        failure-threshold: 3
        success-threshold: 2
        send-on-resolved: true
        description: "shop is down"
- Gatus sends no Content-Type of its own, so the header above is needed. ${SMSTUNNEL_API_KEY} is read from the environment (a literal $ is written $$).
- Placeholders: [ALERT_TRIGGERED_OR_RESOLVED], [ENDPOINT_NAME], [ENDPOINT_GROUP], [ENDPOINT_URL], [ALERT_DESCRIPTION], [RESULT_ERRORS], [RESULT_CONDITIONS]. Only [RESULT_ERRORS] has its quotes escaped, so endpoint names and alert descriptions must not contain double quotes.
- One recipient per request: SMSTunnel sends one text to one number. For several people, one request (or one notification) per number. In Gatus, add a second alert of type custom with provider-override.body for each extra number, or use alerting.custom.overrides per endpoint group.
- failure-threshold / success-threshold decide when it texts; send-on-resolved texts when it is back; minimum-reminder-interval (5m or more) resends while it stays down.
- Answers: 200 {"success": true, "messageId": "…"}; 401 wrong or deleted key; 403 the key lacks a permission ("code": "PERMISSION_REQUIRED"); 400 a missing, unknown or too long field; 429 too many requests (30 sends a minute, with Retry-After): wait and retry; without Retry-After, the plan's limit for the day or month is reached. If my phone is offline the text waits and goes out when it is back, within 3 days.
- The API key comes from my SMSTunnel dashboard (https://dashboard.smstunnel.io): Apps → Add an app → Create the key. It is shown only once and can send texts and check whether they went out.

Rules for your answer:
- Do not ask for my SMSTunnel password or API key and do not put a real key in your answer; I will not paste it in this chat.
- Phone numbers must be in international format (+ and the country code).
- Help me test it with my own number first.
- Only send texts people expect from me. No unsolicited texts.
- Help me pick thresholds so a short blip does not send a text.