ui: fleet update page + endpoints

- POST /api/fleet/update, POST /api/fleet-updates/{id}/cancel,
  GET /api/fleet-updates/{id} (admin-only).
- GET /settings/fleet-update + /partial for htmx polling.
- Renders idle / running / terminal states with per-host progress.
- Tests cover happy path, derive-host-ids, conflict, cancel, get,
  and RBAC.

internal/server/http/server.go

@@ -285,6 +285,11 @@ func (s *Server) routes(r chi.Router) {
 		r.Post("/api/hosts/{id}/update", s.handleHostUpdate)
 		r.Post("/hosts/{id}/update", s.handleHostUpdateForm)
 
+		// Fleet update (P6-15): rolling update across many hosts.
+		r.Post("/api/fleet/update", s.handleAPIFleetUpdateStart)
+		r.Post("/api/fleet-updates/{id}/cancel", s.handleAPIFleetUpdateCancel)
+		r.Get("/api/fleet-updates/{id}", s.handleAPIFleetUpdateGet)
+
 		r.Get("/api/users", s.handleAPIUsersList)
 		r.Post("/api/users", s.handleAPIUserCreate)
 		r.Get("/api/users/{id}", s.handleAPIUserGet)
@@ -298,6 +303,8 @@ func (s *Server) routes(r chi.Router) {
 		if s.deps.UI != nil {
 			r.Post("/hosts/{id}/delete", s.handleUIHostDelete)
 			r.Get("/settings", s.handleUISettings)
+			r.Get("/settings/fleet-update", s.handleUIFleetUpdate)
+			r.Get("/settings/fleet-update/partial", s.handleUIFleetUpdatePartial)
 			r.Get("/settings/users", s.handleUIUsersList)
 			r.Get("/settings/users/new", s.handleUIUserNewGet)
 			r.Post("/settings/users/new", s.handleUIUserNewPost)