fix(ui): a reading's age no longer puts a <time> element inside a title attribute (#5467)

The `when` filter returns a <time> element, so title="{{ at|when }}" wrote raw
markup into the attribute. That broke the hover text, and in the rule form's
metric preview (#5475) it leaked a stray `">` into the page. `ago` already
carries the exact time as its own tooltip, so both places drop the title.
Also fixes #5467's page test, which needed the escaped apostrophe.

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

steward/templates/alerts/_metric_preview.html

@@ -5,7 +5,7 @@
 {% if preview %}
 <div class="metric-preview">
   <span>Now <strong>{{ metric_value(metric, preview.now) }}</strong>
-    <span class="tone-dim" title="{{ preview.at|when }}">{{ preview.at|ago }}</span></span>
+    <span class="tone-dim">{{ preview.at|ago }}</span></span>
   {% if preview.series|length > 1 %}
   <span class="metric-preview-day">
     <span class="tone-dim">last 24 h</span> {{ metric_value(metric, preview.low) }} – {{ metric_value(metric, preview.high) }}