feat(alerts): live-refresh the table every 15s while the tab is visible

The alerts list is the one screen where staleness is genuinely
harmful — an operator can be looking at an Open tab that's already
been resolved by another admin or auto-resolved by the engine, and
take action on a row that no longer exists.

Add an htmx poll on just the table panel:

  hx-get        same URL with current querystring (filters preserved)
  hx-trigger    every 15s, only when document is visible (no idle CPU)
  hx-select     #alerts-table — pull this element out of the response
  hx-swap       outerHTML

Polling lives on the table div, not the page root, so the filter
strip and header don't flash on each tick. Header gains a small
'live ●' label so the polling is discoverable.

RefreshURL is r.URL.RequestURI() on the server side — keeps any
status/severity/host_id/q params intact across refreshes.

Other screens (dashboard, hosts, jobs) deliberately stay manual-
refresh per the project's anti-flicker stance.

web/templates/pages/alerts.html

@@ -90,8 +90,18 @@
     </form>
   </div>
 
-  {{/* alerts table */}}
-  <div class="panel mt-3.5 rounded-[7px] overflow-hidden">
+  {{/* 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. */}}
+  <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-select="#alerts-table"
+       hx-swap="outerHTML">
 
     {{/* header row */}}
     <div class="alert-row head">
@@ -101,7 +111,9 @@
       <div>Message</div>
       <div>Raised</div>
       <div>Last seen</div>
-      <div></div>
+      <div>
+        <span class="text-ink-fade" style="font-size: 10px;" title="auto-refresh every 15s">live ●</span>
+      </div>
     </div>
 
     {{if eq (len $page.Alerts) 0}}