fix: don't set stale timer until two plugin samples establish real interval

Avoids false-stale firing for slow plugins (e.g. nagios_runner at 300 s)
when the heartbeat interval is much shorter. On the first sample cancel
any leftover timer; arm the 3× stale timer only after the second sample.

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

hbd/server/udp.py

@@ -389,10 +389,17 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
                            if k not in ("ID", "plugin", "id", "name")}
             # Store plugin data with timestamp
             host.add_plugin_data(plugin_name, plugin_data, timestamp=now)
-            # Reset stale timer — 3× the heartbeat interval (min 60 s)
-            stale_timeout = max(host.interval * 3, 60)
-            host.reset_plugin_timer(plugin_name, stale_timeout,
-                                    _make_plugin_stale_callback(uname, ctx))
+            # Reset stale timer using the observed send interval for this plugin.
+            # We need two samples to know the real interval; on the first sample
+            # we cancel any leftover timer but don't set a new one, to avoid
+            # false-stale firing for slow plugins (e.g. nagios_runner at 300 s).
+            history = host.plugin_data.get(plugin_name, [])
+            if len(history) >= 2:
+                plugin_interval = max(history[-1][0] - history[-2][0], 1)
+                host.reset_plugin_timer(plugin_name, plugin_interval * 3,
+                                        _make_plugin_stale_callback(uname, ctx))
+            else:
+                host.cancel_plugin_timer(plugin_name)
 
             # If os_info reports an owner and none is configured server-side, apply it
             if plugin_name == "os_info":