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>

steward/templates/base.html

@@ -20,6 +20,7 @@
     <link rel="stylesheet" href="{{ url_for('static', filename='theme.css') }}">
     <link rel="stylesheet" href="{{ url_for('static', filename='steward.css') }}">
     <script src="{{ url_for('static', filename='chart-colors.js') }}"></script>
+    <script src="{{ url_for('static', filename='chart-marks.js') }}"></script>
     <script src="{{ url_for('static', filename='scroll-more.js') }}" defer></script>
     {# A form with data-confirm="…" asks in #confirm-dialog below before it submits (#5314). #}
     <script src="{{ url_for('static', filename='confirm.js') }}" defer></script>