Pin the search in demo mode

Signed-off-by: Hadi <112569860+anotherhadi@users.noreply.github.com>

back/internal/search/manager.go

@@ -131,6 +131,9 @@ func (m *Manager) purgeExpired() {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	for id, s := range m.searches {
+		if s.pinned {
+			continue // demo/pinned searches are never purged
+		}
 		ft := s.FinishedAt()
 		if ft.IsZero() {
 			continue // still running