fix: keep linked event open while source monitor is in ALERT state

Monitor::MonitorLink::score() previously returned 0 whenever the linked
monitor's state was not ALARM, even when its event was still open in
post-event countdown (ALERT). Combined with low AnalysisFPSLimit on the
source monitor (a common setup for HD/sub-stream pairs where the low-res
channel detects motion), state oscillates ALARM <-> ALERT around brief
motion pauses inside a single event. The link signal drops to 0 in those
gaps; the receiving monitor counts down its own PostEventCount and
closes the linked event prematurely — typically after a few seconds,
missing the remainder of a motion event that runs for minutes.

Return a sentinel score of 1 while the linked monitor is in ALERT so the
receiving event stays open. Do not return shared_data->last_frame_score
here: latching the score itself would re-apply a stale numeric to every
subsequent frame, inflating the accumulated score and biasing
snapshot.jpg selection. The sentinel keeps the open-event signal alive
without polluting the score channel.

Real-world evidence: monitor 28 (HD, links to monitor 36 sub-stream)
opened a 5.16-second linked event for a motion sequence that ran 300s
with two motion bursts 134s apart, missing the entire second burst.
With the latch, ALARM->ALERT transitions inside a single source event
are bridged. Long IDLE gaps (separate motion bursts) remain correctly
treated as separate triggers.

src/zm_monitor_monitorlink.cpp

@@ -241,6 +241,14 @@ int Monitor::MonitorLink::score() {
     Debug(1, "Checking all zones score is %d", shared_data->last_frame_score);
     return shared_data->last_frame_score;
   }
+  if (shared_data->state == ALERT) {
+    // Linked monitor's event is still open in its post-event window — motion
+    // has paused but may resume. Return a sentinel so our event stays open;
+    // do not latch last_frame_score, which would inflate the receiving
+    // monitor's accumulated score and bias snapshot.jpg selection.
+    Debug(1, "Linked monitor in ALERT, returning sentinel to keep event open");
+    return 1;
+  }
   Debug(1, "not alarmed. %d", shared_data->state);
   return 0;
 }