## Bug description

The SITE_ENV variable, which is quite useful for PostHog to function properly, is not mentioned in any of the deployment guides and only alluded to in the [Configuring Email](https://posthog.com/docs/configuring-posthog/email) page.

Apparently upgrading from one version of posthog to another broke some links in invite e-mails. I'm not sure, but I assume the e-mail sending code went into an async task, and as of such the URL couldn't be taken from `request` anymore, but reverted to `SITE_URL` that was unset. The e-mail config docs say to set this variable, but it might be easy to skip. Not sure.

## Expected behavior

I'd expect the site to give me a warning if I'm visiting `https://posthog.mysite.com/` while the `SITE_URL` variable is set to `http://localhost:8000/` or even `https://posthog.othersite.com/`. 

In an ideal world, I would see a warning like "You're browsing posthog from https://posthog.newsite.com/ yet your SITE_URL variable is set to https://posthog.oldurl.com/. Would you like to update it?" and a button to update it directly. Alternatively it would send me to a page on the docs that would describe the situation.

## How to reproduce

1. Install PostHog.
2. Either use a webhook or setup e-mail and invite an user (but forget to update SITE_URL)
3. Be confused as to why the link doesn't work.

## Environment

- [ ] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

## Additional context

Was raised in the users' slack.

#### *Thank you* for your bug report – we love squashing them!

posthog/views.py
from ee.clickhouse.system_status import system_status
        metrics.extend(list(system_status()))
    metrics.append({"key": "redis_alive", "metric": "Redis alive", "value": redis_alive})
    if redis_alive:
        import redis
        try:
            redis_info = get_redis_info()
            redis_queue_depth = get_redis_queue_depth()
            metrics.append({"metric": "Redis version", "value": f"{redis_info.get('redis_version')}"})
            metrics.append({"metric": "Redis current queue depth", "value": f"{redis_queue_depth}"})
            metrics.append(
                {"metric": "Redis connected client count", "value": f"{redis_info.get('connected_clients')}"}
            )
            metrics.append({"metric": "Redis memory used", "value": f"{redis_info.get('used_memory_human', '?')}B"})
            metrics.append(
                {"metric": "Redis memory peak", "value": f"{redis_info.get('used_memory_peak_human', '?')}B"}
            )
            metrics.append(
                {
                    "metric": "Redis total memory available",
                    "value": f"{redis_info.get('total_system_memory_human', '?')}B",
                }
            )
        except redis.exceptions.ConnectionError as e:
            metrics.append(
                {"metric": "Redis metrics", "value": f"Redis connected but then failed to return metrics: {e}"}
            )
    return JsonResponse({"results": metrics})
@never_cache
def preflight_check(request: HttpRequest) -> JsonResponse: