feat(syslog): lines beside the readings: around an anomaly, in get_anomaly, and marked on host and SNMP charts (#6002)

get_anomaly's circumstances gain the host's or SNMP device's syslog
warnings and worse (mcp_events / mcp_resource_events). An anomaly row opens
"Syslog around it", read only when opened. A shared Chart.js plugin
(chart-marks.js) draws a dashed line at each syslog error on the host
metrics charts and an SNMP device's charts, with a line saying how many.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/snmp/templates/snmp/device.html

@@ -42,6 +42,7 @@
 
 {# The device's own log, when it sends syslog to an added source (#6000). #}
 {% if "syslog" in enabled_plugins %}
+{% if charts %}<p id="snmp-marks-note" class="field-hint" hidden style="margin:0.5rem 0 0;"></p>{% endif %}
 <div hx-get="/plugins/syslog/panel?address={{ device.address|urlencode }}{% if device.host_id %}&amp;host_id={{ device.host_id }}{% endif %}" hx-trigger="load" hx-swap="innerHTML"></div>
 {% endif %}
 
@@ -56,6 +57,7 @@
       ? d.toLocaleDateString([], { weekday: "short", day: "numeric" }) + " " + d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit", hour12: false })
       : d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit", hour12: false });
   }
+  var drawn = [];
   charts.forEach(function (c, i) {
     var el = document.getElementById("snmp-c-" + i);
     if (!el) return;
@@ -63,7 +65,7 @@
       el.parentElement.innerHTML = '<p class="field-hint" style="margin:0;">Not enough readings yet for a chart.</p>';
       return;
     }
-    new Chart(el.getContext("2d"), {
+    drawn.push(new Chart(el.getContext("2d"), {
       type: "line",
       data: { labels: c.points.map(function (p) { return p[0]; }),
               datasets: [{ data: c.points.map(function (p) { return p[1]; }), borderColor: line,
@@ -81,8 +83,13 @@
                title: { display: !!c.axis, text: c.axis, font: { size: 9 } } }
         }
       }
-    });
+    }));
   });
+  {% if "syslog" in enabled_plugins %}
+  // The device's own syslog errors over the same span (#6002).
+  if (drawn.length) stewardMarkCharts("/plugins/syslog/marks?address={{ device.address|urlencode }}{% if device.host_id %}&host_id={{ device.host_id }}{% endif %}&hours=" + hours,
+                                      drawn, document.getElementById("snmp-marks-note"));
+  {% endif %}
 })();
 </script>
 {% endblock %}