Revert emailVerified check in EmailDomainAccessPolicy to preserve
backward compatibility with GitHub/GitLab OAuth users. Instead, null
unverified emails in CustomOidcUserService.toOAuthClaims() so
EmailDomainAccessPolicy naturally denies them via null email.
Add SLF4J logging to CustomOidcUserService for OIDC authentication
flow tracing and failure diagnostics.
Add registration ID collision warning to deployment docs.