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.

internal/server/http/ui_alerts.go

@@ -14,10 +14,11 @@ import (
 )
 
 type alertsPage struct {
-	Filter    store.AlertFilter
-	Alerts    []store.Alert
-	Counts    alertCounts
-	HostNames map[string]string // host_id → name for table rendering
+	Filter     store.AlertFilter
+	Alerts     []store.Alert
+	Counts     alertCounts
+	HostNames  map[string]string // host_id → name for table rendering
+	RefreshURL string            // self-URL for the live-refresh poll
 }
 
 type alertCounts struct {
@@ -51,7 +52,12 @@ func (s *Server) handleUIAlerts(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 		return
 	}
 
-	page := alertsPage{Filter: f, Alerts: alerts, HostNames: map[string]string{}}
+	page := alertsPage{
+		Filter:     f,
+		Alerts:     alerts,
+		HostNames:  map[string]string{},
+		RefreshURL: r.URL.RequestURI(),
+	}
 	if hosts, err := s.deps.Store.ListHosts(r.Context()); err == nil {
 		for _, h := range hosts {
 			page.HostNames[h.ID] = h.Name