feat: OAuth independent registration

[wehub4me]

Summary

Implements #8042 - OAuth independent registration functionality.

Changes

• Add oauth_registration_enabled setting to allow OAuth users to register even when general registration is disabled
• Add oauth_only setting to restrict users to OAuth-only authentication
• Add oauth_only field to users table to mark OAuth-only users
• Update OauthController to respect new settings
• Add UI controls in Advanced settings for admin configuration

Issues

• fix [Enhancement]: OAUTH only self registering. #8042

Category

• Feature

AI Assistance

• I used AI assistance in this PR

Testing

• Tested OAuth registration logic
• Tested OAuth-only enforcement

Contributor Agreement

• I have read and understood the contributor guidelines. If I have failed to follow any guideline, I understand that this PR may be closed without review.
• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate.
• I have tested all the changes thoroughly with a local development instance of Coolify and I am confident that they will work as expected when a maintainer tests them.