The closest that I know, and open source too, is either Git or https://github.com/dsnopek/anki-sync-server
According to advertising, I am tempted to use Firestore, but
- Serverless web app cannot store too many images in IndexedDB. (Nor it is efficient)
I also tried PouchDB / CouchDB, which can also store images as attachments, but I noticed that user account control is missing.