Implement model instance count for each retrieve_ command

[tlrh314]

The data can fluctuate considerably due to scrapers / add_products implementation in the Cece pipeline. So we may want to implement a simple model that holds a timestamp, content_type_pk, and instance pk, and count.

We can then use these timeseries data to monitor the number of certificates/brands/stores/products in our database, and set limits for warnings (e.g. if one Store/Brand suddenly drops from 100 to 40 products).