style: clear whitespace, unused-import and blank-line lint

Strip trailing whitespace, drop unused imports/variables/global statements,
split multi-imports, fix E3xx/E1xx spacing and E712 comparisons. No behaviour
change; only E501 long lines remain in flake8.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/server/monitor.py

@@ -1,15 +1,13 @@
 """Monitor helper for heartbeat daemon.
 
 This module provides monitoring tasks for the heartbeat daemon.
-The primary reachability monitoring is now event-driven (timers set/reset 
+The primary reachability monitoring is now event-driven (timers set/reset
 on HTB arrival in udp.py) rather than periodic polling.
 
 This module can be extended for additional monitoring tasks.
 """
 
 from __future__ import annotations
-import asyncio
-import time
 from . import notify as notify_mod
 
 DROPOVERDUE = 7 * 24 * 3600
@@ -18,11 +16,10 @@ eventlog = notify_mod.eventlog
 
 async def cleanup_connections(hbdclass):
     """Clean up connection timers on shutdown.
-    
+
     Cancels all active overdue timers to prevent callbacks after shutdown.
     """
     for hostname, host in list(hbdclass.Host.hosts.items()):
         for conn_type, conn in host.connections.items():
             if hasattr(conn, 'cancel_overdue_timer'):
                 conn.cancel_overdue_timer()
-