PgVectorEmbedding objects out of sync with Embeddings

The PgVector backend uses its own model where embeddings are stored for indexing by `pgvector`.

It seems in some cases these models can get out of sync with the source `Embedding` models:

```
>>> PgvectorEmbedding.objects.all().count()
850
>>> Embedding.objects.all().count()
887
```

Cause to be investigated