Seed last_seen from ARP cache on startup so watchdog has a baseline

Without this, last_seen is empty after every restart and the watchdog
skips all devices (ts is None → continue). Devices that are genuinely
active refresh last_seen via ARP requests or mDNS. AP proxy ghosts only
send ARP replies (now filtered), so they go silent and the watchdog
fires departure after WATCHDOG_TIMEOUT_SEC.

net_alerter/net_alerter.py

@@ -821,6 +821,12 @@ def seed_from_arp_cache():
                     if ip in infrastructure_ips:
                         dev['infrastructure'] = True
                     known_devices[mac] = dev
+                    # Seed last_seen so the watchdog has a baseline.
+                    # Devices that are genuinely active will refresh this via ARP
+                    # requests or mDNS; AP proxy ghosts (only ARP replies) won't.
+                    with last_seen_lock:
+                        if mac not in last_seen:
+                            last_seen[mac] = time.time()
 
         logging.info(f"Seeded {len(known_devices)} devices from ARP cache")
     except Exception as e: