Report devices seen on last scan in ARP/SNMP status

The ARP and SNMP scanner status endpoints and front-end cards now expose
the number of devices found by the most recent successful scan, following
the existing mDNS device-count pattern. The count persists across the
running/waiting transitions, and for SNMP a failed poll keeps the last
good count rather than overwriting it.

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

backend/src/scanners/arp/scanner.rs

@@ -22,6 +22,7 @@ pub async fn scan() -> Result<(), Box<dyn std::error::Error>> {
 
         info!("Done with ARP probes");
         info!("Found {} online devices", devices.len());
+        status::record_scan(devices.len() as u64);
 
         // Process found devices
         for device in devices.iter() {