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/syslog/templates/syslog/_around.html

@@ -0,0 +1,25 @@
+{# plugins/syslog/templates/syslog/_around.html — an anomaly's syslog: the
+   warnings and worse nearest its start, from the sources shown with its
+   resource (#6002). Loaded when its "Syslog around it" is opened. #}
+{% from "syslog/_tone.html" import tone %}
+{% if not sources %}
+<p class="field-hint" style="margin:0.3rem 0 0;">No added syslog source is shown with this resource.</p>
+{% elif not lines %}
+<p class="field-hint" style="margin:0.3rem 0 0;">No warnings or worse from {% for s in sources %}{% if not loop.first %}, {% endif %}<a href="/plugins/syslog/sources/{{ s.id }}">{{ s.name }}</a>{% endfor %} from half an hour before to half an hour after.</p>
+{% else %}
+<table class="table" style="font-size:0.78rem;margin-top:0.3rem;">
+  <thead><tr><th class="nowrap">Received ({{ tz_label() }})</th>{% if sources|length > 1 %}<th>Source</th>{% endif %}<th>Severity</th><th>App</th><th>Message</th></tr></thead>
+  <tbody>
+  {% for ln in lines %}
+  <tr>
+    <td class="nowrap">{{ ln.received_at|when(seconds=true) }}</td>
+    {% if sources|length > 1 %}<td class="nowrap"><a href="/plugins/syslog/sources/{{ ln.source_id }}">{{ ln.source }}</a></td>{% endif %}
+    <td class="nowrap"><span class="{{ tone(ln.level) }}">{{ ln.severity }}</span></td>
+    <td class="nowrap">{{ ln.app or "—" }}</td>
+    <td class="mono" style="max-width:32rem;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;" title="{{ ln.message }}">{{ ln.message }}</td>
+  </tr>
+  {% endfor %}
+  </tbody>
+</table>
+{% if more > 0 %}<p class="field-hint" style="margin:0.3rem 0 0;">{{ more }} more in that hour either side, further from its start: open the source to read them.</p>{% endif %}
+{% endif %}