Count passive scanner devices seen in the last hour

Replace the lifetime "devices seen since start" counter in the mDNS, SSDP
and DHCP scanners with a rolling count of distinct devices (deduped by
MAC) seen within the last hour.

Each scanner's status now tracks a MAC -> last-seen-time map; the snapshot
prunes entries older than 60 minutes and reports the remaining count. The
API field name (devices_seen) is unchanged, so only its meaning and the
frontend labels ("N devices in the last hour") are updated.

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

backend/src/scanners/ssdp/scanner.rs

@@ -123,5 +123,5 @@ async fn process_announcement(
         }
     }
 
-    status::record_discovery();
+    status::record_discovery(&mac);
 }