Fix departure detection for AP ARP-proxy networks (passive-only)

ARP proxy on WiFi APs causes the kernel neighbor table to show devices
as REACHABLE even after they disconnect. This made last_seen refresh
continuously from on_arrival() (netlink path), preventing the watchdog
from ever detecting departure.

Two fixes:
1. Remove _update_last_seen() from on_arrival() — netlink events are
   not reliable as a genuine-traffic source on ARP-proxy networks.
   last_seen is now only updated by the raw packet capture path (ARP
   requests, mDNS, DHCP) where src_mac must be the device itself.

2. Watchdog now covers all is_personal_device() entries in known_devices,
   not just the personal_devices enrollment set. device_labels entries
   (like named phones) were previously unmonitored by the watchdog.

net_alerter/net_alerter.py

@@ -953,9 +953,6 @@ def update_occupancy_state() -> None:
 
 def on_arrival(mac: str, ip: str, hostname: str = "") -> None:
     """Handle device arrival."""
-    # Update passive observation tracking
-    _update_last_seen(mac)
-    
     # Infrastructure IPs never trigger alerts
     if ip in infrastructure_ips:
         logging.debug(f"Ignoring infrastructure IP {ip} (MAC:{mac})")
@@ -1758,9 +1755,9 @@ def personal_watchdog() -> None:
             time.sleep(60)  # Check every minute
             now = time.time()
             
-            # Get current set of personal devices without holding lock for long
-            with personal_lock:
-                tracked = set(personal_devices)
+            # Track all personal devices currently in known_devices (not just enrolled set)
+            with known_lock:
+                tracked = {mac for mac in known_devices if is_personal_device(mac)}
 
             for mac in tracked:
                 # Get last_seen timestamp