Consolidate active-scan notifications and scrub private data

Active scanners (ARP, SNMP) now accumulate every change across a whole
scan and emit one notification per type via events::notify: a single
device produces the usual single-device notification (carrying its MAC),
while two or more produce one consolidated summary with an empty
mac_address. Device events are still recorded per device.

Notification bodies no longer include MAC or IP addresses; the title
MAC fallback is masked to the last two octets. Summaries list up to
three devices then "…and N more devices".

Split sighting handling so record_sighting persists + records the event
and returns Vec<DeviceChange>; passive listeners (mDNS, SSDP, DHCP) use
record_and_notify since they see one device per event.

Also fixes NotificationType::from_str never mapping "DeviceChanged",
which made those notifications round-trip from the DB as Other.

Frontend: the card already hides the device link when mac_address is
null; added widget tests for the present/absent link cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/scanners/ssdp/finder.rs

@@ -193,7 +193,10 @@ mod tests {
                     USN: uuid:abcd::upnp:rootdevice\r\n\r\n";
         let announcement = parse_announcement(body.as_bytes()).expect("alive packet");
         assert!(announcement.server.is_none());
-        assert_eq!(announcement.device_types, vec!["upnp:rootdevice".to_string()]);
+        assert_eq!(
+            announcement.device_types,
+            vec!["upnp:rootdevice".to_string()]
+        );
     }
 
     #[test]