What does this PR do?

This PR introduces a granular control toggle for OAuth-based user registration.

Currently, OAuth registration is tied to the general registration setting. This change allows administrators to disable public registration (via email/password) while still allowing new users to register through trusted OAuth providers (Google, GitHub, Authentik, etc.).

Changes:

• Migration: Added is_oauth_registration_enabled to the instance_settings table.
• Logic: Updated OauthController to check both general and OAuth-specific registration toggles.
• UI: Added the “OAuth Registration Allowed” toggle to the Settings > Advanced view.
• Tests: Added a feature test to verify the new registration logic.

Why this is needed:

Many organizations want to restrict access to their Coolify instance to only those who can authenticate via a corporate SSO (OAuth), but want to prevent random email/password signups.

Screenshots:

(Screenshots showing the new toggle in Advanced Settings)

Fixes #8042

/claim #8042