fix: threshold and logging improvements

- threshold: fix crash when display is None (_format_display now falls
  back to default format string instead of calling None.format())
- threshold: shorten notification messages by stripping plugin-name prefix
  from metric_path (cpu_percent instead of cpu_monitor.cpu_percent)
- main: demote aiohttp.access log records from INFO to DEBUG
- udp: replace debug print with proper logger.info for new host sign-on

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/main.py

@@ -475,6 +475,7 @@ def run(config, config_path=None):
     if config.get("debug", 0) > 0:
         log_level = logging.DEBUG
     logging.basicConfig(level=log_level)
+    logging.getLogger("aiohttp.access").setLevel(logging.DEBUG)
     load_pickled_hosts(config, hbdclass)
 
     notify_mod.initlog(logfile=config.get("logfile", "messages.log"))