stabilize id generation
Currently the ids for the items are created automatically (I guess an auto_increment-function works in the background). If we do a new ingest (includes dropping the old database) items may get a different id as they had before (e.g. if you run the PH ingestion before the Tapor ingestion, PH items do get the first numbers; doing it the opposite way the Tapor items will get the first numbers). In the end we do need persistent ids. So when we go public we either need to guarantee, that a complete new ingest will deliver the same ids for items as before or we then can't do a complete new ingest. It is important that at a certain moment we freeze the ids for the already available items.