Send app screens from your own server

Set up An app with its own server for Sumly in this project.

Sumly (https://sumly.dev) shows revenue next to the visits and app users it came from. The full guide is https://sumly.dev/install/app-server; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors › App users and make the project's secret key (`sk_sumly_…`). It is shown once: keep it in your server's environment as `SUMLY_KEY`.
2. From your server, send `POST https://sumly.dev/api/v1/events` with the key as a Bearer token, up to 1,000 events in one call, each with the person's RevenueCat app user id as `user_id`.
3. Check the key with `GET /api/v1/ping`, then see the events arrive on the project's Sources list.

JSON: With the secret key, send the country you know for the person; Sumly cannot take it from your server's address.
```
POST https://sumly.dev/api/v1/events
Authorization: Bearer sk_sumly_…
Content-Type: application/json

{
  "events": [
    { "type": "screen", "screen": "Paywall", "user_id": "u_42", "platform": "ios", "country": "DE" },
    { "type": "track", "name": "trial_started", "user_id": "u_42", "platform": "ios" }
  ]
}
```

Keep in mind: The secret key stays on the server. A key that sits in an app or a browser is the public one (`sl_…`), which can send events only. Each event is checked on its own: the good ones are kept and the reply lists the refused ones and why.

How to work:
- Look at how this project is built first, and fit the code to its framework and its way of doing things; the examples above are a starting point, not something to paste as is.
- Steps that happen in a dashboard (Sumly, or An app with its own server's own) are mine: tell me exactly what to click, then wait for me.
- Keys the code needs belong in environment variables, never in the code or the repository. Never ask me to paste a secret key into this chat; tell me which variable to set.
- Change only what this needs, show me the changes before you commit, and tell me how to check that it works.