Fixes #8042

/claim #8042

Summary

• Adds a new is_oauth_registration_enabled setting to instance settings, allowing admins to permit new users to self-register via OAuth providers even when general (password-based) registration is disabled
• When enabled, users authenticating through configured OAuth providers (Authentik, Azure, GitHub, etc.) can create accounts automatically, while the standard registration form remains disabled
• Adds an “OAuth Registration Allowed” toggle in Settings > Advanced, right below the existing “Registration Allowed” checkbox

Changes

• Migration: Added is_oauth_registration_enabled boolean column (default: false) to instance_settings table
• OauthController: Updated registration gate check — new OAuth users are allowed if either is_registration_enabled OR is_oauth_registration_enabled is true
• Advanced settings (Livewire + Blade): Added the new setting to the component properties, validation rules, mount/save logic, and UI

Testing

• With both settings disabled: OAuth login for new users returns 403 (unchanged behavior)
• With only is_oauth_registration_enabled enabled: new OAuth users can register, but the standard /register form stays disabled
• With is_registration_enabled enabled: all registration works as before (unchanged behavior)
• Existing OAuth users can always log in regardless of either setting