fix: clear plugin data and timers on connection UP transition

Moves the plugin-state purge from the boot flag to the UP transition,
so stale history and alerts are cleared on any reconnect (reboot, or
recovery from overdue/unknown) not just detected reboots.

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

hbd/server/udp.py

@@ -477,11 +477,23 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
                 notify_mod.Notification(title=f"[INFO] {uname}", body=f"{host.name} booted", level="INFO"),
             ))
     if message:
-        eventlog(uname, "INFO", "msg: %s" % message, service=service)
+        eventlog(uname, "INFO", message, service=service)
 
     if conn.getstate() != hbdcls.Connection.UP:
         lasts = conn.state
         d = conn.newstate(hbdcls.Connection.UP, now)
+        # On reboot, pre-boot plugin data and derived alerts are stale.
+        # Cancel all plugin timers and wipe plugin state so timers restart
+        # cleanly from the first two post-boot samples.
+        for pname in list(host.plugin_timers):
+            host.cancel_plugin_timer(pname)
+        host.plugin_data.clear()
+        stale_plugin_keys = [
+            k for k in host.alert_states
+            if k not in ("rtt",) and not k.startswith("connectivity.")
+        ]
+        for k in stale_plugin_keys:
+            del host.alert_states[k]
         # Clear connectivity alert now that the host is back up
         _set_connectivity_alert(host, conn.afam, "OK")
         # Don't log/notify RECOVER for a brand-new host seen for the first time —