feat(alerts): a rule can fire when a reading rises, "rose by N" (#6724)

A rule compared a reading with a fixed number, so Synology Disk 1's bad
sectors going from 3,160 to 3,392 could say nothing: a count that only grows
is soon past any line. "rose by" compares each reading with the one before it,
kept on the alert's state and only for such a rule. A fall (a replaced disk)
doesn't fire. On the rule form, through MCP, and worded in emails, the alert
sentence, history and the timeline.

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

steward/templates/alerts/rules_form.html

@@ -102,12 +102,13 @@
       </div>
       <div data-cond-numeric {% if binary %}hidden{% endif %} class="form-pair">
         <div class="form-group">
-          <label>Fires when the reading is</label>
+          <label>Fires when the reading</label>
           <select name="operator" {% if binary %}disabled{% endif %}>
             {% for val, label in operators %}
             <option value="{{ val }}" {% if rule and rule.operator.value == val %}selected{% endif %}>{{ label }}</option>
             {% endfor %}
           </select>
+          <p class="field-hint">"rose by" compares each reading with the one before it, for a count that only grows. A reading that hasn't risen counts as OK, so "Good checks in a row before it clears" sets how long the alert stays up.</p>
         </div>
         <div class="form-group">
           <label>Threshold <span data-threshold-unit style="color:var(--text-muted);font-weight:normal;">{% if metric_unit(m0) %}({{ metric_unit(m0) }}){% endif %}</span></label>