fix: suppress recover messages for down durations under 4 seconds

Transient blips caused by hbc client restarts no longer generate
eventlog entries or notifications.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

hbd/server/udp.py

@@ -440,8 +440,12 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         if not newh:
             if d == 0 or lasts == "unknown":
                 m = "%s is up" % (conn.afam)
+            elif d < 4:
+                # Transient blip (likely client restart) — skip log and notification
+                m = None
             else:
                 m = "%s back after being %s for %s" % (conn.afam, lasts, dur(d))
+            if m:
                 eventlog(uname, "RECOVER", m)
                 if host.watched:
                     asyncio.create_task(notify_mod.send_notification(