Make default role configurable instead of hardcoded

[Amyti]

Problem

The default role used during migration is currently hardcoded.
This causes issues when redeploying, especially when the role already exists
or when using custom role names.

Proposed solution

Allow configuring the role via:

• an environment variable (e.g. AUTH_DEFAULT_ROLE)
• or a config file option

Benefits

• Easier redeployments
• Better flexibility for different environments
• Avoids manual DB intervention

Context
This issue appears when running migrations on redeploy.