So that consumers of the events API can tell whether a sighting came
from the ARP scanner or the mDNS listener. Adds a DeviceEventScanner
enum (Arp / Mdns) plumbed from each scanner's call site through to a
new scanner column on device_events, exposed via the existing events
endpoint and OpenAPI schema. Pre-existing rows are backfilled with
'ARP' since that was the only scanner before mDNS.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
