Add interface flap detection and env var for infrastructure IPs in net_alerter

- Fix 1: Interface flap detection suppresses departure storms when wlan0 briefly loses association and triggers simultaneous RTM_DELNEIGH events for all cached neighbors
  - Tracks 3-second window of departure MACs
  - Locks recovery state for 60 seconds if 3+ departures occur in rapid succession
  - Prevents false positive alerts during interface recovery

- Fix 2: Additional infrastructure IPs via NET_ALERTER_INFRA_IPS env var
  - Allows operator to suppress alerts for devices like access points that cycle regularly
  - Comma-separated list of IP addresses to be seeded as infrastructure at startup
  - Example: NET_ALERTER_INFRA_IPS=10.0.0.0,10.0.0.0

Addresses DevTrack #467 (net_alerter false positive alerts)

net_alerter/net_alerter.py

@@ -37,6 +37,37 @@ departure_timers = {}  # {mac: threading.Timer} for 15-min departure debounce
 departure_lock = threading.Lock()  # Protects departure_timers and last_departed_time
 last_departed_time = {}  # {mac: timestamp} when device was actually alerted as departed
 
+# Interface flap detection — suppresses departure storms
+_flap_window = []          # [(timestamp, mac)]
+_flap_lock = threading.Lock()
+_interface_recovering = False
+_recovery_timer = None
+
+
+def _check_flap(mac: str) -> bool:
+    """Return True if this departure is part of an interface flap (suppress it)."""
+    global _interface_recovering, _recovery_timer
+    now = time.time()
+    with _flap_lock:
+        _flap_window[:] = [(t, m) for t, m in _flap_window if now - t < 3.0]
+        _flap_window.append((now, mac))
+        if len(_flap_window) >= 3 and not _interface_recovering:
+            _interface_recovering = True
+            if _recovery_timer:
+                _recovery_timer.cancel()
+            _recovery_timer = threading.Timer(60.0, _clear_recovery)
+            _recovery_timer.daemon = True
+            _recovery_timer.start()
+            logging.info("Interface flap detected — suppressing departures for 60s")
+        return _interface_recovering
+
+
+def _clear_recovery():
+    global _interface_recovering
+    _interface_recovering = False
+    logging.info("Interface flap recovery window ended")
+
+
 
 # Top 200 common device OUI prefixes (MAC first 3 octets)
 OUI_DICT = {
@@ -505,6 +536,11 @@ def on_departure(mac: str) -> None:
             logging.debug(f"Ignoring departure for infrastructure IP {dev['ip']} (MAC:{mac})")
             return
 
+        # Check if this is part of an interface flap (suppress simultaneous mass-departure)
+        if _check_flap(mac):
+            logging.debug(f"Flap suppression: ignoring departure for {mac}")
+            return
+
         dev_copy = dev.copy()
         # Mark departing instead of popping — allows re-arrival to detect and cancel timer
         known_devices[mac]['departing'] = True
@@ -627,6 +663,12 @@ def seed_infrastructure_ips(iface: str) -> None:
 
     logging.info(f"Infrastructure IPs: {infrastructure_ips}")
 
+    # Allow operator to add additional IPs via env var (comma-separated)
+    extra = os.environ.get("NET_ALERTER_INFRA_IPS", "")
+    for ip in (x.strip() for x in extra.split(",") if x.strip()):
+        infrastructure_ips.add(ip)
+        logging.info(f"Seeded extra infrastructure IP {ip} from env")
+
 
 def dhcp_sniffer(iface: str) -> None:
     """