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/udp.py

@@ -336,8 +336,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         # Apply user-access settings from config
         access = config_mod.get_host_access(cfg, uname)
         host.apply_access(access["owner"], access["managers"], access["monitors"])
-        if verbose:
-            print(("XX: New host, num now %s" % (len(hbdcls.Host.hosts))))
+        logger.info("New host signed on: %s (dyn=%s, access=%s)", uname, host.dyn, access)
         newh = True
     else:
         host = hbdcls.Host.hosts[uname]