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/web_server/dhcp_scanner.rs

@@ -9,7 +9,7 @@ pub struct DhcpScannerStatusResponse {
     pub is_listening: bool,
     /// Seconds the listener has been running (only set when is_listening is true)
     pub listening_for_seconds: Option<f64>,
-    /// Total device requests processed since the listener started
+    /// Distinct devices seen in the last hour
     pub devices_seen: u64,
     /// Seconds since the last device was seen (None if none seen yet)
     pub last_device_seen_seconds_ago: Option<f64>,
@@ -51,7 +51,7 @@ pub async fn status() -> Result<Json<DhcpScannerStatusResponse>, StatusCode> {
     Ok(Json(DhcpScannerStatusResponse {
         is_listening: snapshot.is_listening,
         listening_for_seconds,
-        devices_seen: snapshot.devices_discovered,
+        devices_seen: snapshot.devices_last_hour,
         last_device_seen_seconds_ago,
     }))
 }