alerts: 5s polling cadence + live toggle + severity colour cues

Two operator-visible changes on /alerts:

1. Polling drops from 15s to 5s and gains a checkbox in the table
   header to turn live monitoring on/off. Choice is persisted in
   localStorage so it survives full-page navigations. The toggle
   state is woven into the htmx hx-trigger predicate, so flipping
   the checkbox just sets the flag and the next tick (or the
   absence of one) honours it — no attribute juggling, no
   htmx.process re-init. The dot dims to 0.3 opacity when paused
   so operators can see at a glance that they're looking at a
   stale view.

2. Severity dropdown options pick up the same oklch tints used by
   the row dots / left borders / kind chips. The kind column shows
   only the kind text, so without a colour cue the dropdown
   mentioned a concept (severity) that the table itself didn't
   render. Now the colours bridge the gap.

Note on <option> styling: Chrome and Firefox honour inline color:
on options; Safari ignores it. Acceptable degradation — falls back
to plain text, which is what we had.

web/templates/pages/alerts.html

@@ -56,14 +56,17 @@
       {{end}}
     </div>
 
-    {{/* severity dropdown */}}
+    {{/* severity dropdown — option text tinted to match the colour
+         already used in the row (dot, left border, kind chip). The
+         severity word is otherwise invisible to operators because the
+         table column shows kind only; the colour bridges the two. */}}
     <div>
       <select class="field" style="padding: 6px 10px; font-size: 11.5px; min-width: 130px;"
               onchange="window.location='/alerts?status={{$filter.Status}}&severity='+this.value+'{{if $filter.HostID}}&host_id={{$filter.HostID}}{{end}}{{if $filter.Search}}&q={{$filter.Search}}{{end}}'">
         <option value="" {{if eq $filter.Severity ""}}selected{{end}}>Severity · any</option>
-        <option value="info"     {{if eq $filter.Severity "info"}}selected{{end}}>info</option>
-        <option value="warning"  {{if eq $filter.Severity "warning"}}selected{{end}}>warning</option>
-        <option value="critical" {{if eq $filter.Severity "critical"}}selected{{end}}>critical</option>
+        <option value="info"     style="color: oklch(0.78 0.005 250);" {{if eq $filter.Severity "info"}}selected{{end}}>● info</option>
+        <option value="warning"  style="color: oklch(0.82 0.13 80);"   {{if eq $filter.Severity "warning"}}selected{{end}}>● warning</option>
+        <option value="critical" style="color: oklch(0.70 0.20 25);"   {{if eq $filter.Severity "critical"}}selected{{end}}>● critical</option>
       </select>
     </div>
 
@@ -90,16 +93,16 @@
     </form>
   </div>
 
-  {{/* alerts table — polled every 15s while the tab is visible.
-       hx-get re-fetches the same URL (so filter querystring is preserved)
-       and hx-select pulls just this element out of the full response,
-       replacing the live one. Pauses automatically when the tab is
-       backgrounded so we're not burning CPU on inactive tabs.
-       The polling lives here (not on the page root) so the filter strip
-       and header don't flash on each tick. */}}
+  {{/* alerts table — polled every 5s when the tab is visible AND the
+       live toggle is on. The localStorage check is part of the htmx
+       trigger predicate, so flipping the toggle just sets the flag and
+       the next tick (or the absence of one) honours it. No need to
+       re-process the element when the toggle changes.
+       The polling lives on this div (not the page root) so the filter
+       strip and header don't flash on each tick. */}}
   <div id="alerts-table" class="panel mt-3.5 rounded-[7px] overflow-hidden"
        hx-get="{{$page.RefreshURL}}"
-       hx-trigger="every 15s [document.visibilityState==='visible']"
+       hx-trigger="every 5s [document.visibilityState==='visible' && localStorage.getItem('rm-alerts-live')!=='off']"
        hx-select="#alerts-table"
        hx-swap="outerHTML">
 
@@ -111,8 +114,15 @@
       <div>Message</div>
       <div>Raised</div>
       <div>Last seen</div>
-      <div>
-        <span class="text-ink-fade" style="font-size: 10px;" title="auto-refresh every 15s">live ●</span>
+      <div style="display: flex; align-items: center; gap: 6px; justify-content: flex-end;">
+        <label style="display: inline-flex; align-items: center; gap: 5px; cursor: pointer; font-size: 10px;"
+               class="text-ink-fade" title="auto-refresh every 5s">
+          <input type="checkbox" id="alerts-live-toggle" checked
+                 onchange="localStorage.setItem('rm-alerts-live', this.checked ? 'on' : 'off'); document.getElementById('alerts-live-dot').style.opacity = this.checked ? '1' : '0.3';"
+                 style="width: 11px; height: 11px; cursor: pointer; margin: 0;" />
+          <span>live</span>
+          <span id="alerts-live-dot" class="text-accent">●</span>
+        </label>
       </div>
     </div>
 
@@ -138,4 +148,26 @@
   </div>
 
 </div>
+
+<script>
+// Restore the live-refresh toggle from localStorage so the operator's
+// last choice survives full-page navigations. Re-runs after every htmx
+// swap so the freshly-rendered checkbox + dot stay in sync.
+(function syncLiveToggle() {
+  var on = localStorage.getItem('rm-alerts-live') !== 'off';
+  var cb = document.getElementById('alerts-live-toggle');
+  var dot = document.getElementById('alerts-live-dot');
+  if (cb) cb.checked = on;
+  if (dot) dot.style.opacity = on ? '1' : '0.3';
+})();
+document.body.addEventListener('htmx:afterSwap', function(e) {
+  if (e.detail.target && e.detail.target.id === 'alerts-table') {
+    var on = localStorage.getItem('rm-alerts-live') !== 'off';
+    var cb = document.getElementById('alerts-live-toggle');
+    var dot = document.getElementById('alerts-live-dot');
+    if (cb) cb.checked = on;
+    if (dot) dot.style.opacity = on ? '1' : '0.3';
+  }
+});
+</script>
 {{end}}