PostgreSQL
Connect your own database as a read-only data source, over a public address you control.
The connection string is encrypted at rest and never returned by the API — you get a masked hint, not the password back. The role you give it should be read-only; Ragenta will refuse a query that tries to write regardless.
Nominate which tables an agent may see. Everything else in the database stays invisible to it, including the columns you would rather no model ever summarised.