Delay arrival alert 2s so mDNS hostname resolves before alert fires

DHCP triggers on_arrival immediately but iPhone sends mDNS frames
milliseconds later. Without the delay the alert fires with no hostname
and falls back to the device label. Timer reads from known_devices at
fire time so mDNS-updated hostname is used.

net_alerter/net_alerter.py

@@ -1089,10 +1089,24 @@ def on_arrival(mac: str, ip: str, hostname: str = "") -> None:
             'last_seen': now
         }
 
-    msg = format_arrival(hostname, ip, vendor or "unknown", mac)
-    logging.info(msg)
-    send_alert(msg)
-    update_occupancy_state()
+    # Delay alert 2 seconds so mDNS can update the hostname before we send.
+    # The timer callback reads from known_devices at fire time to get the real name.
+    def _send_arrival_alert():
+        with known_lock:
+            dev = known_devices.get(mac)
+            if dev is None:
+                return  # Device departed before timer fired
+            resolved_hostname = dev.get('hostname', hostname)
+            resolved_ip = dev.get('ip', ip)
+            resolved_vendor = dev.get('vendor', vendor)
+        msg = format_arrival(resolved_hostname, resolved_ip, resolved_vendor or "unknown", mac)
+        logging.info(msg)
+        send_alert(msg)
+        update_occupancy_state()
+
+    t = threading.Timer(2.0, _send_arrival_alert)
+    t.daemon = True
+    t.start()
 
 
 def on_departure(mac: str) -> None: