fix(alerts): a rule's page doesn't wait for its metric preview (#5485); Send test up top (#5486)

#5485 "A rule's page waits up to 25 s for its metric preview: a cold 24 h
series read from plugin_metrics". Opening "Agent offline" took 25.6 s; a
cold day of one series measured 18.3 s over MCP, a warm one 63 ms. The
pickers still come with the page from plugin_metric_latest; the preview is
its own request (hx-trigger=load), showing "Reading the last day…" until it
lands. Picker swaps no longer read it either.

#5486 "Send test where it can be found": the button moves from inside "What
happens" to the page header beside History, its result under the verdict.

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

steward/templates/alerts/_metric_preview.html

@@ -13,6 +13,8 @@
   </span>
   {% endif %}
 </div>
+{% elif metric and resource and not preview_read %}
+<p class="field-hint">Reading the last day…</p>
 {% elif metric and resource %}
 <p class="field-hint">No reading of this yet from {{ resource }}.</p>
 {% endif %}