Align MongoDB configuration with postgres and mysql patterns:
- Add separate `databases` section with `roles.admin` and `roles.readonly`
- Simplify `users` to only contain optional password field
- All users now authenticate via `admin` database (MongoDB best practice)
- Admin role maps to readWrite + dbAdmin permissions
- Readonly role maps to read permission
Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>
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