Record originating scanner on device events

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>

backend/database_migrations/09-add_scanner_to_device_events/up.sql

@@ -0,0 +1 @@
+ALTER TABLE device_events ADD COLUMN scanner TEXT NOT NULL DEFAULT 'ARP' COLLATE NOCASE;