feat(http): host mode toggle handler + route (host.mode_updated)

internal/server/http/ui_handlers.go

@@ -983,6 +983,43 @@ func (s *Server) handleUIHostTagsSave(w stdhttp.ResponseWriter, r *stdhttp.Reque
 	stdhttp.Redirect(w, r, "/hosts/"+hostID, stdhttp.StatusSeeOther)
 }
 
+// handleUIHostModeSave flips a host's always-on flag. Checkbox present
+// in the form (value any) => always-on; absent => intermittent.
+// Operator-band; mounted in server.go. On change we clear open
+// offline/staleness alerts via the engine so the next sweep re-raises
+// only what still applies under the new mode.
+func (s *Server) handleUIHostModeSave(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	u := s.requireUIUser(w, r)
+	if u == nil {
+		return
+	}
+	hostID := chi.URLParam(r, "id")
+	if _, err := s.deps.Store.GetHost(r.Context(), hostID); err != nil {
+		stdhttp.NotFound(w, r)
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		stdhttp.Error(w, "bad request", stdhttp.StatusBadRequest)
+		return
+	}
+	alwaysOn := r.PostForm.Get("always_on") != ""
+	if err := s.deps.Store.SetHostAlwaysOn(r.Context(), hostID, alwaysOn); err != nil {
+		slog.Error("ui host mode: save", "host_id", hostID, "err", err)
+		stdhttp.Error(w, "internal", stdhttp.StatusInternalServerError)
+		return
+	}
+	if s.deps.AlertEngine != nil {
+		s.deps.AlertEngine.ResolveOnModeChange(r.Context(), hostID, time.Now().UTC())
+	}
+	_ = s.deps.Store.AppendAudit(r.Context(), store.AuditEntry{
+		ID: ulid.Make().String(), UserID: &u.ID, Actor: "user",
+		Action:     "host.mode_updated",
+		TargetKind: ptr("host"), TargetID: &hostID,
+		TS: time.Now().UTC(),
+	})
+	stdhttp.Redirect(w, r, "/hosts/"+hostID, stdhttp.StatusSeeOther)
+}
+
 // normaliseTags splits a comma-separated string, lowercases each token,
 // trims whitespace, drops empties, and dedupes. Order is preserved
 // from first occurrence (so the user's typing order shows on screen).