Self-hosted Inngest logs flooded with "rejecting event; event key not recognized"

## The symptom

Self-hosted Inngest (docker compose, `inngest start`) with `INNGEST_EVENT_KEY`
and `INNGEST_SIGNING_KEY` set. The server boots fine and functions run, but the
logs are flooded every minute with:

```
rejecting event; event key not recognized (caller=api)
```

## Why

This flood is the Inngest server's own usage telemetry, not your app. The
telemetry client POSTs to the server's own event API with a hardcoded telemetry
event key (pkg/api/tel/tel.go) that the server does not recognize, so each send
is rejected and retried ~8 times with backoff, producing bursts of error lines
every minute.

## The fix

Set `DO_NOT_TRACK=1` in the Inngest server's environment. The telemetry client
checks it and skips sending entirely. Confirmed in the thread: logs went from
~15 errors/minute to zero.

## Do not confuse this with the app-side version

The same error from your app usually means `INNGEST_EVENT_KEY` is not set on
both the server and your SDK. The distinguishing signal: if your functions run
fine and the flood is constant and minute-shaped, it is the telemetry, not your
key.