A model version must be unique by hash

We cannot have multiple versions sharing the same hash:

• add a unique together on ModelVersion: model + hash
• the endpoint CreateModelVersion must not allow to create a new version if a version with same hash exists on the model. If such a match exists, the endpoint must return 400 and different payloads:
  • an error when the match has state == available
  • the normal payload with s3_put_url otherwise (so we can recover uploads in case of system errors)