Sign-out of an OIDC session redirects the browser to the provider's
discovered end_session_endpoint (id_token_hint + absolute
post_logout_redirect_uri) when PHOTOPRISM_OIDC_LOGOUT is enabled, so the
provider SSO session ends and the next login re-prompts. Opt-in (default
off); local/LDAP logout and providers without an end-session endpoint fall
back to local logout.
Adds the shared CE /api/v1/oauth/logout route and OAuthLogoutHandler hook
plus the Portal OP end_session_endpoint advertised in the Portal discovery
document, for Portal builds to serve via the override hook.
