Fixes #8042

This PR adds a new setting is_oauth_registration_enabled that allows admins to permit new users to self-register via OAuth providers even when general password-based registration is disabled.

Changes

• Migration: Added is_oauth_registration_enabled column to instance_settings table.
• Advanced Settings: Added a new “OAuth Registration Allowed” toggle in Settings > Advanced.
• OauthController: Updated registration logic to check for either general registration OR OAuth-specific registration.

/claim #8042