feat(alerts): pick the host, then its part, and see what the metric reads (#5475)

#5475 "Rule form: pick the host, then its part; metrics that part reports;
a link to groups; a preview of the metric", from the operator's review of
the New rule page: hosts and their parts shared one long picker.

- Host agent rules pick a Host, then a Part: "The whole host", then its
  mounts, CPU cores, disks, interfaces and sensors under a heading each,
  cores in number order. A part that reports nothing a rule can watch
  (an interface's traffic) isn't offered.
- The Metric list holds what the picked resource reports, in catalog order.
- Under it, a preview: the reading now, and the last 24 h low to high with
  an hourly sparkline drawn from zero (UI ruling 8), read from one series.
- The group picker links to Ansible > Inventory to make a group, in a new
  tab, and re-reads the groups when the tab comes back into view.
- Picking a host or part swaps the pickers alone (/alerts/api/rule_target);
  picking a metric swaps the preview (/alerts/api/metric_preview).
- A rule's own resource and metric stay on offer after they go quiet.

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

steward/templates/alerts/_rule_fields.html

@@ -1,8 +1,10 @@
-{# alerts/_rule_fields.html — the resource and metric pickers for one source;
-   the rule form includes it and swaps it in again when the source changes.
-   For a source whose resources are hosts, the rule can instead cover every
-   host, or every host in an inventory group (#5464): Steward then keeps one
-   rule per host under it. What a rule covers is fixed once it exists. #}
+{# alerts/_rule_fields.html — what a rule watches, for one source; the rule
+   form includes it and swaps it in again when the source changes. For a
+   source whose resources are hosts, the rule can instead cover every host,
+   or every host in an inventory group (#5464): Steward then keeps one rule
+   per host under it. What a rule covers is fixed once it exists. The host,
+   part and metric pickers are alerts/_rule_target.html, swapped on their
+   own as a host or part is picked (#5475). #}
 {% set covers = current_covers|default("resource") %}
 {% set one = covers == "resource" %}
 {% if host_source|default(false) %}
@@ -16,7 +18,7 @@
   <select id="rule-covers" name="covers" data-covers>
     <option value="resource" {% if one %}selected{% endif %}>One host, or one part of a host</option>
     <option value="all_hosts" {% if covers == "all_hosts" %}selected{% endif %}>Every host</option>
-    <option value="group" {% if covers == "group" %}selected{% endif %} {% if not groups %}disabled{% endif %}>Every host in a group{% if not groups %} (no inventory groups yet){% endif %}</option>
+    <option value="group" {% if covers == "group" %}selected{% endif %}>Every host in a group</option>
   </select>
   {% endif %}
   <p class="field-hint">Covering hosts keeps one alert per host under this rule, each named
@@ -25,43 +27,17 @@
 </div>
 <div class="form-group" data-cover-group {% if covers != "group" %}hidden{% endif %}>
   <label for="rule-group">Group</label>
-  <select id="rule-group" name="cover_group" {% if covers != "group" %}disabled{% endif %}>
-    {% for g in groups %}
-    <option value="{{ g }}" {% if g == current_group|default("") %}selected{% endif %}>{{ g }}</option>
-    {% endfor %}
-  </select>
-  <p class="field-hint">A host is in a group through its linked Ansible target (Ansible → Inventory).</p>
+  {# Re-read when the reader comes back to this tab, so a group just made in
+     Inventory is here without losing what was typed. #}
+  <div id="rule-group-picker" hx-get="/alerts/api/cover_groups" hx-trigger="visibilitychange[document.visibilityState === 'visible'] from:document"
+       hx-include="[name='cover_group'],[name='covers']" hx-swap="innerHTML">
+    {% include "alerts/_cover_groups.html" %}
+  </div>
+  <p class="field-hint">A host is in a group through its linked Ansible target.
+    <a href="/ansible/inventory/?add=group" target="_blank" rel="noopener">Make a group, or change who is in one, in Ansible → Inventory</a>;
+    it shows here when you come back.</p>
 </div>
 {% endif %}
-<div class="form-group" data-cover-resource {% if not one %}hidden{% endif %}>
-  <label>Resource</label>
-  {% if resources %}
-  <select name="resource_name" required {% if not one %}disabled{% endif %}>
-    {% if not current_resource %}<option value="" disabled selected>— pick a resource —</option>{% endif %}
-    {% for r in resources %}
-    <option value="{{ r }}" {% if r == current_resource %}selected{% endif %}>{{ r }}</option>
-    {% endfor %}
-  </select>
-  {% else %}
-  <select name="resource_name" required disabled data-none="1">
-    <option value="">Nothing has reported from this source yet</option>
-  </select>
-  <p class="field-hint">
-    The list fills from readings this source has recorded. Come back once it has reported.
-  </p>
-  {% endif %}
-</div>
-
-<div class="form-group">
-  <label>Metric</label>
-  <select name="metric_name" required data-metric-picker>
-    {% if not current_metric %}<option value="" disabled selected>— pick a metric —</option>{% endif %}
-    {% for m in metrics %}
-    <option value="{{ m }}" data-unit="{{ metric_unit(m) }}" data-about="{{ metric_about(m) }}" {% if m in ("is_up", "is_healthy") %}data-binary{% endif %}
-            {% if m == current_metric %}selected{% endif %}>{{ metric_label(m) }}</option>
-    {% endfor %}
-  </select>
-  {% if current_metric and metric_about(current_metric) %}
-  <p class="field-hint" data-metric-about>{{ metric_about(current_metric) }}</p>
-  {% else %}<p class="field-hint" data-metric-about hidden></p>{% endif %}
+<div id="rule-target">
+{% include "alerts/_rule_target.html" %}
 </div>