Migrate search to Vertex AI and enhance preview modal

backend/handlers/api.go

@@ -67,6 +67,7 @@ func (h *Hub) Remove(conn *websocket.Conn) {
 type PreviewResponse struct {
 	Title           string            `json:"title"`
 	Thumbnail       string            `json:"thumbnail"`
+	PreviewStreamURL string           `json:"previewStreamUrl"`
 	Duration        string            `json:"duration"`
 	DurationSeconds int               `json:"durationSeconds"`
 	StartDefault    string            `json:"startDefault"`