Add flag to disable local auth

[acelinkio]

• I have read this document: https://miniflux.app/opinionated.html#feature-request

I would like to request a feature that allows disabling local authentication within the application. My homelab relies on an OIDC identity provider to manage all aspects of authentication, including:
Multi-Factor Authentication (MFA)
Password complexity requirements
Password rotation policies

Removing local authentication support will enhance the security by ensuring that all authentication processes are handled by my OIDC identity provider. This not only streamlines security protocols but also minimizes potential vulnerabilities associated with maintaining multiple authentication mechanisms.

[fguillot]

See #2752