POST /api/v1/beacon-monitor/measurements accepts one measurement per
integration window from a propmonitor client, authenticated by the
BeaconMonitor token. Records noise floor, signal peak/avg dBFS, SNR,
signal-active-fraction, gain, and frequency for later correlation
against weather and propagation scores.
Beacon UUID must resolve to an approved + on-the-air beacon (404
otherwise — the client drops 404s without retry). Retries with the
same (monitor, beacon, measured_at) are idempotent: a unique index
makes the second insert a 409. Every accepted upload stamps the
monitor's last_seen_at — measurements *are* the heartbeat.
MonitorAuth is a separate plug from the user API-token Auth plug;
monitors are not users. The rate limiter buckets each monitor by id so
retry storms after a 5xx don't burn the shared anon-IP bucket.
Users can now register beacon monitors on the settings page. Each
monitor gets a unique random token the remote program will use to
authenticate its reports. Name is the only user-supplied field for
now; more will be added as the monitor protocol is defined.
Also adds :gen_smtp to deps. Swoosh's SMTP adapter depends on
:mimemail which lives in that package, and production was 500'ing
when trying to send confirmation emails without it.
