Add a read-only user to postgres for grafana

[AiroPi]

Curenlty, Grafana use the same credentials than the bot to read data from PostgreSQL. However, this is not a good practice and not recommended, because it can execute SQL query that can unintentionally alter the datas.

Alembic can be used to create a new migration executing CREATE USER with only SELECT permissions.
The user could be named GRAFANA.

I don't know much about postgres users, but I can help if needed.