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/src/web_server.rs

@@ -1,6 +1,6 @@
 use std::error::Error;
 
-use crate::model::device_events::{DeviceEvent, DeviceEventType};
+use crate::model::device_events::{DeviceEvent, DeviceEventScanner, DeviceEventType};
 use crate::model::devices::{Device, DeviceSummary};
 use crate::model::notifications::{Notification, NotificationType};
 use crate::settings::get_settings;
@@ -63,6 +63,7 @@ pub mod utils;
         UpdateDevicePayload,
         DeviceEvent,
         DeviceEventType,
+        DeviceEventScanner,
         ArpScannerStatusResponse,
         MdnsScannerStatusResponse,
     )),